Webapp2

----------------------------------------------------
- A default charset is always set in the Request object: if it is not defined
in the constructor, environ['CONTENT_TYPE'] is checked; and if it is still
not defined 'utf-8' is used.

- Added token_model as an attribute of the User model; it is used instead of
the hardcoded class.

--------------------------------
- Fixed a bug in WSGIApplication.run() when running outside of App Engine:
CGIHandler().run(app) -> CGIHandler().run(self). (issue 9)

===========================
- Revised webapp2_extras.auth and simplified User model, by Kyle Finley.
Thanks!

===========================
- Added webapp2_extras.auth: high-level utilities for authentication and
authorization.

- Added webapp2_extras.appengine.auth.models: default model implementations
for webapp2_extras.auth (can be overridden).

- Added webapp2_extras.appengine.ndb.unique_model: a model to store unique
values. This was added mainly so that the default user model can have
multiple unique properties.

- import_string() now displays useful debug information when it fails.

- To make things clearly separated specially for those using webapp2
outside of App Engine, from now on all App Engine-specific modules will
be placed in webapp2_extras.appengine. Also three modules were moved to
the new package:

- webapp2_extras.sessions_memcache
- webapp2_extras.sessions_ndb
- webapp2_extras.users

The original modules were not removed, though: they will be kept in the same
place for compatibility purposes, just importing their stuff from the new
locations.

- Config.load_config() now makes a copy of the configuration when user_values
are passed. That keyword is meant to override app configuration, so the
previous behavior was actually wrong. This is a side feature not used inside
webapp2 or even well documented so the change should not affect existing
apps.

=============================
- cached_property now uses threading.RLock for thread-safety.

=============================
- Little fix: use google.appengine.ext.webapp.util if available, like before.