fix(tests): add tests for Django 1.2 and 1.3

This fix simply adds the testing configuration for Django 1.2 and 1.3
and removes the tox configuration for python2.5 as that is no longer
supported since v3.1.

The extra command for Django 1.2 simply cleans up the temporary database
created by the testsuite in Django 1.2.
fix(tests): fix database setup for Django < 1.4

This fixes the TypeError that occurs during the database setup for the
testsuite in Django < 1.4. Removing some of the class attributes allows
for the 'inner()' method to be called from the BaseField 'instance'
instead of the PickedObjectField.

Example error:
TypeError: unbound method inner() must be called with BaseField instance
as first argument (got PickledObjectField instance instead)
fix(tests): fix deprecated imports for Django <1.6

This fixes an import that breaks in older versions of Django. It's just
the 'django.conf.urls.defaults' import that was deprecated and removed
in Django 1.6.
fix(tests): fix CACHE_BACKEND test for Django 1.2

The format for the CACHE_BACKEND Django setting was changed in Django
1.3+ to the new dictionary-based CACHES style (which used to be a single
string-based setting i.e. scheme://). This fix simply updates the
CACHE_BACKEND setting in the test so it operates as expected for Django
1.2.