Release v1.6rc1

[jambonrose]

I'm opening a pull-request to document the changes made for release v1.6rc1. I intend to merge and release a new version immediately (provided PyPI will let me).

These changes :

• update documentation and metadata,
• provide general cleanup:
  • remove unnecessary condition for Django 1.4
  • adds iSort as import formatter
  • upgrade the version of unittest-mixins
• add a test of the package (Manifest.in in particular)

Each commit is structured as a distinct change to make review as easy as possible, so I will let those act as change-log.

I ran the Travis build via my repo. All Tox tests pass locally.