Adds to Melange a tags framework based on taggable-mixin. The taggable-mixin allowed only tag per Datastore model. This is extended framework allows any arbitrary number of tags per Datastore model. Also, now one can define different models for different Tag types which are all inherited from the base Tag model provided by taggable-mixin. The GHOPTask model makes use of 2 tags per model, one for difficulty and the other for task_type, both using the tags framework. Reviewed by: Paweł Sołyga

"""
Field-like classes that aren't really fields. It's easier to use objects that
have the same attributes as fields sometimes (avoids a lot of special casing).
"""

from django.db.models import fields

class OrderWrt(fields.IntegerField):
    """
    A proxy for the _order database field that is used when
    Meta.order_with_respect_to is specified.
    """
    name = '_order'
    attname = '_order'
    column = '_order'