Implement "client" pattern for datastore

Per #861.

Steps:

Rename gcloud.datastore.dataset.Dataset -> gcloud.datastore.client.Client. (Rename 'dataset.Dataset' -> 'client.Client'. #946)
Unwind _implicit_environ usage within client. (Move implicit majyk inside 'Client.__init__'. #956)
Move gcloud.datastore. 7E1F api functions to methods of Client. (Move 'datastore.api' functions to 'datastore.client.Client' methods. #963)
Update Client to handle nesting of batches / transactions over primary connection. (Update 'Client' to hold stack of connection/batch/transaction objects. #976)
Update domain objects making API requests to hold reference to client. (Update batch/transaction/query to hold client. #978)
Update Batch and Transaction context managers to push themselves onto their client's stack. (Update batch/transaction/query to hold client. #978)
Update domain object methods taking connection to accept client, falling back to self.client. (Update batch/transaction/query to hold client. #978)
Remove remaining _implicit_environ usage. (Remove '_implicit_environ' #979)

Provide feedback