Add new 'class Meta' paradigm.

Class Meta will take over many of the roles of providing customization
for form behavior and feature configuration. In addition, it will also
allow better ability for complementary libraries to hook into WTForms
without needing to monkeypatch.
Add documentation for class Meta.
Make version clear it's an API break
ext.appengine: Remove deprecated label_attr
Start adding deprecation warnings to extensions.
Add coveralls to travis
No babel for python3 yet.
Test babel on python2 and pypy
Update deprecationwarning
Start getting more on top of class meta.
stop using LOCALES, instead use Meta.locales.
Add in core CSRF (work in progress)
Start testing core CSRF functionality.
Clean up session CSRF
Don't coerce attr=false to string
Don't use descriptors for default Meta values.

Unfortunately, with multiple inheritance, and also potentially with
allowing assignment post-construction and with keywords, having
csrf_class or any other such property be a descriptor causes headaches.
It's much easier to simply move to a factory pattern instead.
Enable caching translations.
Improve documentation of Meta
Better documentation for class Meta.
Fields keep a reference to the form meta