v0.6 prep and bugfix

@apollo13,

Found a little time to bang this out. 

The first commit covers the bugfix for reverse relationships (related to #80) with tests.

I also updated the project version to 0.6a1 and added a CHANGES.rst to start keeping track of the changes between releases - didn't bother to backlog any details from before this upcoming release though, but can if you think it necessary.

Not sure how you'd like to handle noting contributions going forward... there's the AUTHORS.txt file, /alex/django-filter/contributors page, plus the issues and pull-requests.

Finally, I updated the MANIFEST.in to account for the new files that have been added to the project, but you should probably take a look to confirm we want to package all of those.