Version 1.2

I tried to merge improvements and bug fixes from many different forks, I reviewed the features, wrote tests and removed the things which weren't relevant.

The main improvements are the followings:

* no need for a custom backend anymore
* support for postgis
* query optimization in ReferenceField
* nice admin widget for django-grappelli
* django 1.5.5 support
* improved docs
* added more tests
* added project to continuos integration testing with travis-ci.org