Support for mapping\filtering entities in ndb connection field

[davidis1]

This PR contains support for post NDB query interference to achieve mapping and filtering of entities fetched by NDB queries resolved within NDBConnectionFields.

It's very useful in situations where there's a many to many relation between two NDB models and you need to paginate over them.
See the added tests for an example.

Example explained -
A new Reader model was added. Each reader may have read multiple articles, and each article may have been read by multiple readers. The many to many relation is implemented using the ArticleReader model which represents that some reader has read an article and contains the reader key and article key.
Now, in order to paginate over all the article readers, the readers property was added to ArticleType:
readers = NdbConnectionField(ReaderType, entities_mapper=readers_mapper, entity_filter=reader_filter)
Notice that ArticleType's resolve_readers returns a query for all the ArticleReader entities.
Upon resolving it readers_mapper is called and maps each article reader to the appropriate reader.
Later we filter the readers in reader_filter and only return the readers that are alive.

[davidis1]

@ekampf please review.

[coveralls]

Coverage decreased (-0.05%) to 91.711% when pulling 05eb67a on davidis1:feature/mapping_entities_in_connection_field into f886661 on graphql-python:master.

[coveralls]

Coverage increased (+0.07%) to 91.826% when pulling 40d0409 on davidis1:feature/mapping_entities_in_connection_field into f886661 on graphql-python:master.