Implement "client" pattern for datastore

[tseaver]

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.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)

[dhermes]

Thanks for the outline!

[dhermes]

@tseaver I am going to factor out Client as a base class in core. Will this cause problems for you?

[tseaver]

@dhermes go ahead for pubsub / storage, but please leave the one in datastore alone until after I'm done here.

[dhermes]

#955 is just in place as an example, I dropped the don't merge label on it.

I'd like to get #953 in before starting on storage. pubsub is all done (aside from the refactoring)