Support custom "patched" version strings

Modified the way that Django reports version number. You can now
specify "patched" in the third position of the VERSION tuple in
django/__init__.py. For example:

    VERSION = (1, 4, 5, 'patched', 0)

Django will report a version of:

    1.4.5-patched-0

This is so we can specify a custom version of Django as a dependency
in our setup.py