Adding pragma: no cover statements for AppEngine imports.
Model adapter for in-memory backend (for use in testing / single-threaded deployments). Added more tests and ensured proper 'no cover' pragma's. Converted indentation to spaces.
Cleaned up lots of comment blocks, converted more spacing from tabs to spaces, added testsuite for InMemoryAdapter.
Fixed issues with new inmemory testsuite.
Updates to Model API - model parent property classes can now be cloned during inheritance (bugfix), model subclasses now properly inherit data descriptors (bugfix), classes extending `model.adapter.abstract.KeyMixin` or `model.adapter.abstract.ModelMixin` are automatically registered and added to each `AbstractModel`/`AbstractKey` concrete class' MRO chain (new feature), MRO hook-in ability via `MetaFactory`-subclasses, general bugfixes, updates to testsuite, fixed operation of class-level property decorator (`decorators.classproperty`, bugfix).