Fixes for new app engine release (import endpoints and ResourceContainer)

1.  I changed all references of `google.appengine.ext.endpoints` to the new endpoints library. Also updated the examples accordingly.

2.  First attempts at implementing `ResourceContainer`.

    For `query_method` I create a `ResourceContainer` with `message_types.VoidMessage` as request body
    and using the provided `query_fields`

    For normal methods I check for path parameters and if they exist create a `ResourceContainer` using the
    Model itself (respecting `request_fields`) as request body and adding the extracted path parameters.

I've tested this with one of my more advanced Endpoint APIs and it works without problems and warnings.

Biggest problem I see is with Message Classes now all being `CombinedContainer` so checking against the cached proto_models won't work. Not sure if there is a good solution for that (or if the check is really necessary).

This should fix https://github.com/GoogleCloudPlatform/endpoints-proto-datastore/issues/79