Adds a way to set page_version_fn in settings.
Adds a way to set the encryptation function in settings.
Adds notes in the README about the possibility to configure page version and encryptation methods.
Sets the base_cache_key to 255 chars, once we are not assuming this will be always md5 hexdigest anymore.
Adds a middleware that sets the page version cookie based in request, when it is customized.
Documents that the PageVersionCookieMiddleware is useful even if the cache already saves the version in a cookie.
Check for custom encryptation and page version functions at runtime instead of the first time loaded. This will let me monkey-patch the settings in tests.
Created tests for custom page version and encryptation functions.
Migrations for base_cache_key max_length update.
Merge branch 'master' into configurable_cachekey
Creates Django 1.7 migrations and move South ones to another folder.

Follow django recommendations:
https://docs.djangoproject.com/en/1.7/topics/migrations/#libraries-third-party-apps
Suggests migrate instead of syncdb, with South or Django 1.7 own method.