Product variants with collections (and autocomplete)

I suggest to discard previous pull request for this feature - #130 . In this branch I did some cleanup and squashed small commits. 

This branch includes:
* Product variants with sizes and collections
* Product collections are implemented as ```CharField(db_index=True)``` (as @patrys suggested)
* Product collections have auto-complete feature implemented with django-selectable (works on Python 3)
* All hints from previous PR are implemented and errors are fixed

We should merge this to master and later we can merge currency conversion (#133)