Datastore NDB support

[romanlevin]

Support for the Google App Engine NDB service would be really awesome.

[LuRsT]

We'll be happy to consider a patch for that.

[brianmhunt]

Here is a legacy issue regarding this: code.google.com:fixture#58

[hello-josh]

HI! I know this is old but we use fixture with Google App Engine and I am porting over the details of a custom EntityMedium we use to handle db and ndb entities at the same time.

Here is the initial commit: hello-josh@e9f280d

It seems that the Google App Engine tests are so old that they will no longer run. I am going to refactor those as well then make a PR.