Adapt gaek for "namespace_manager" availability

[erichiggins]

As noted in #20 :

Based on an examination of the migration notes, it appears that there is at least one change which would break gaek, specifically the removal of google.appengine.api.namespace_manager which has no replacement.

The migration notes state:

There is no replacement for google.appengine.api.namespace_manager which is used to determine the default namespace when not passed in to Key()

This seems to primarily affect envrion.py which imports namespace_manager and makes its methods available.