Django-user-tags

- Added migrations

- Prepared app for Django 1.10 and Python 3

- added admin classes for the app.

- BACKWARDS INCOMPATIBLE CHANGE!! You need to change the definition of your tag
fields on your model. It is now a dict of dicts, not a list of dicts.
- mixin now adds one `fieldname_tags_values` method for each tags field to the
form. This allows to easily retrieve the current values in your template.

- added setting USER_TAGS_RELATED_NAME with default `user_tags_tagged_items` to
avoid name clashes with `django-tagging`.

- BACKWARDS INCOMPATIBLE CHANGE: TAG_FIELDS is no longer a list of 3-tuples
but a list of dictionaries. See README for more info. As a result we now
support to define a help_text for the tags form field.