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

"""
The md5 and sha modules are deprecated since Python 2.5, replaced by the
hashlib module containing both hash algorithms. Here, we provide a common
interface to the md5 and sha constructors, preferring the hashlib module when
available.
"""

try:
    import hashlib
    md5_constructor = hashlib.md5
    sha_constructor = hashlib.sha1
except ImportError:
    import md5
    md5_constructor = md5.new
    import sha
    sha_constructor = sha.new