Add Python 3 compatibility

Hi,

This PR is initiation for Python 3 support. I hope this will allow a Py3 experimental release.

I tried to follow http://python3porting.com and https://docs.djangoproject.com/en/dev/topics/python3/ and it uses `six` compatibility library.

All the tests are passing in both 2 and 3, except few glitches. Mainly, tests with 'assertRedirects' fail because in Python 3 version, the response redirect url is correct but parameters are not in order. I have only used redirect status code for Py3 for now, hoping that others or you might know better about the issue. 

The other notable issue was, while running tests on Py3, it complains about database table name. I am not sure exactly why, thus I set the db name `FACEBOOK_OG_SHARE_DB_TABLE = 'django_facebook_open_graph_share'` and it's working fine. 

Please review :)