Add documentation (sphinx)

Issue: https://github.com/clearcode/pytest-dbfixtures/issues/12

Preview: http://pytest-dbfixtures.readthedocs.org/en/latest/

I think we should wait to this pull request: https://github.com/clearcode/pytest-dbfixtures/pull/11

After that we should remove conflicts and we can merge :)

After merge:
* change default branch from docs to master (http://readthedocs.org)