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

# Getting the normal admin routines, classes, and `site` instance.
from django.contrib.admin import autodiscover, site, StackedInline, TabularInline, HORIZONTAL, VERTICAL

# Geographic admin options classes and widgets.
from django.contrib.gis.admin.options import GeoModelAdmin
from django.contrib.gis.admin.widgets import OpenLayersWidget

try:
    from django.contrib.gis.admin.options import OSMGeoAdmin
    HAS_OSM = True
except ImportError:
    HAS_OSM = False