move send_file and send_from_directory to Werkzeug

The implementations were moved to Werkzeug, Flask's functions become
wrappers around Werkzeug to pass some Flask-specific values.

cache_timeout is renamed to max_age. SEND_FILE_MAX_AGE_DEFAULT,
app.send_file_max_age_default, and app.get_send_file_max_age defaults
to None. This tells the browser to use conditional requests rather than
a 12 hour cache.

attachment_filename is renamed to download_name, and is always sent if
a name is known.

Deprecate helpers.safe_join in favor of werkzeug.utils.safe_join.

Removed most of the send_file tests, they're tested in Werkzeug.

In the file upload example, renamed the uploaded_file view to
download_file to avoid a common source of confusion.

tests/test_blueprints.py

@@ -222,7 +222,7 @@ def test_templates_and_static(test_apps):
         assert flask.render_template("nested/nested.txt") == "I'm nested"
 
 
-def test_default_static_cache_timeout(app):
+def test_default_static_max_age(app):
     class MyBlueprint(flask.Blueprint):
         def get_send_file_max_age(self, filename):
             return 100