Ported to support Django 1.6, in which the underlying caching API has
changed. See https://docs.djangoproject.com/en/dev/releases/1.6/#miscellaneous.
To support both Django 1.6 and < 1.6 simultaniously, a new constant,
DEFAULT_TIMEOUT, was added to replace the use of the None timeout value.
This is consistent with how caching works in Django 1.6.
Part of this porting also fixes #44.

Updated the documentation accordingly, and added some points on changing
timeout values, including NO_CACHE.
Corrected incorrect default value for CACHE_COUNT_TIMEOUT.
Updated travis config for 1.6.
Merge pull request #48 from groklearning/django-1.6

Django 1.6 support