Added lazyloading pattern and explicit chapter links in tutorial.

This fixes #49.

docs/patterns/index.rst

@@ -28,3 +28,4 @@ Snippet Archives <http://flask.pocoo.org/snippets/>`_.
    flashing
    jquery
    errorpages
+   lazyloading

docs/patterns/lazyloading.rst

@@ -0,0 +1,104 @@
+Lazily Loading Views
+====================
+
+Flask is usually used with the decorators.  Decorators are simple and you
+have the URL right next to the function that is called for that specific
+URL.  However there is a downside to this approach: it means all your code
+that uses decorators has to be imported upfront or Flask will never
+actually find your function.
+
+This can be a problem if your application has to import quick.  It might
+have to do that on systems like Google's AppEngine or other systems.  So
+if you suddenly notice that your application outgrows this approach you
+can fall back to a centralized URL mapping.
+
+The system that enables having a central URL map is the
+:meth:`~flask.Flask.add_url_rule` function.  Instead of using decorators,
+you have a file that sets up the application with all URLs.
+
+Converting to Centralized URL Map
+---------------------------------
+
+Imagine the current application looks somewhat like this::
+
+    from flask import Flask
+    app = Flask(__name__)
+
+    @app.route('/')
+    def index():
+        pass
+
+    @app.route('/user/<username>')
+    def user(username):
+        pass
+
+Then the centralized approach you would have one file with the views
+(`views.py`) but without any decorator::
+
+    def index():
+        pass
+
+    def user(username):
+        pass
+
+And then a file that sets up an application which maps the functions to
+URLs::
+
+    from flask import Flask
+    from yourapplication import views
+    app = Flask(__name__)
+    app.add_url_rule('/', view_func=views.index)
+    app.add_url_rule('/user/<username>', view_func=views.user)
+
+Loading Late
+------------
+
+So far we only split up the views and the routing, but the module is still
+loaded upfront.  The trick to actually load the view function as needed.
+This can be accomplished with a helper class that behaves just like a
+function but internally imports the real function on first use::
+
+    from werkzeug import import_string, cached_property
+
+    class LazyView(object):
+
+        def __init__(self, import_name):
+            self.__module__, self.__name__ = import_name.rsplit('.', 1)
+            self.import_name = import_name
+
+        @cached_property
+        def view(self):
+            return import_string(self.import_name)
+
+        def __call__(self, *args, **kwargs):
+            return self.view(*args, **kwargs)
+
+What's important here is is that `__module__` and `__name__` are properly
+set.  This is used by Flask internally to figure out how to do name the
+URL rules in case you don't provide a name for the rule yourself.
+
+Then you can define your central place to combine the views like this::
+
+    from flask import Flask
+    from yourapplication.helpers import LazyView
+    app = Flask(__name__)
+    app.add_url_rule('/',
+                     view_func=LazyView('yourapplication.views.index'))
+    app.add_url_rule('/user/<username>',
+                     view_func=LazyView('yourapplication.views.user'))
+
+You can further optimize this in terms of amount of keystrokes needed to
+write this by having a function that calls into
+:meth:`~flask.Flask.add_url_rule` by prefixing a string with the project
+name and a dot, and by wrapping `view_func` in a `LazyView` as needed::
+
+    def url(url_rule, import_name, **options):
+        view = LazyView('yourapplication.' + import_name)
+        app.add_url_rule(url_rule, view_func=view, **options)
+
+    url('/', 'views.index')
+    url('/user/<username>', 'views.user')
+
+One thing to keep in mind is that before and after request handlers have
+to be in a file that is imported upfront to work propery on the first
+request.  The same goes for any kind of remaining decorator.