Remove datastore.Dataset class

[dhermes]

The replacement will be handled by a string when dataset ID is explicitly needed (which will only be for power users). Some functionality will be moved to key.Key and the rest to the connection module as free functions. In order to make use easier, they will also have versions in __init__ which can use a default / implicit connection.

Steps to complete:

• Make set_default_connection similar to set_default_dataset (Address first part of 477: Implements datastore.set_default_connection. #478)
• Remove query(), entity(), and transaction() methods from Dataset, replace their use elsewhere. (Address second part of 477: Remove query(), entity(), and transaction() methods from Dataset. #479)
• Move Dataset.get_entity to Key (will impact elsewhere, e.g. Entity.reload) (Address third part of 477: Move Dataset.get_entity to Key. #480)
• Move save() and delete() methods delete() method from Entity to Key (Address fourth part of 477: Moving Entity.delete to Key (for datastore). #481)
• Move allocate_ids() method from Dataset to connection __init__ module as free function (Address fifth part of 477: Moves Dataset.allocate_ids to connection module. #482)
• Move get_entities() method from Dataset to connection __init__ module as free function (Address sixth part of 477: Moves Dataset.get_entities to __init__ module. #483)
• Fix Remove dataset from Entity constructor and allow a key #484, Remove Query dependency on Dataset and make dataset_id required in Query #485, Remove Transaction dependency on Dataset #486 - Lots to do to re-make Entity, Query, and Transaction to not depend on Dataset
• Remove all remnants of Dataset (e.g. set_default_dataset, Entity._must_dataset, and _implicit_environ._DatastoreBase)
• Update stale documentation

NOTE: Don't close until cyclic-import in #480 is resolved.

See https://docs.google.com/document/d/1xKw8Tz6lfN5uOcv0my4uUgFSeDoRzwCLho2UGlBpqDU/edit for more.