'Constructor for Storage.
Args:
model: db.Model, model class
key_name: string, key name for the entity that has the credentials
property_name: string, name of the property that is a CredentialsProperty
cache: memcache, a write-through cache to put in front of the datastore'

@util.positional(4)
def __init__(self, model, key_name, property_name, cache=None):

    self._model = model
    self._key_name = key_name
    self._property_name = property_name
    self._cache = cache
