This updates djblets.cache, djblets.datagrid, and djblets.gravatar to be
compatible with newer versions of Django. These include pulling in some
backwards-compatibility functions and updating to use the proper APIs.

In the case of datagrids, the profile-related code has finally been
removed. I've changed the logic to no longer assume the existence of
User.get_profile(), and instead to allow subclasses to override a
function that returns some object (a profile model, or something else)
for use in data storage.

All unit tests pass on Django 1.6 and 1.8.