Support for Django 1.6+

Rewritten most of the code of the tags, now should be compatible with:

• Django 1.6+, 1.7
• 1.8 and 1.9 should be fine too.

I've tested this version on Django 1.9, so I presume should be fine on the other versions too.

Lots of stuff fixed on the tags, also added some decent documentation.