Adding api.delete() and removing Key.delete().

[dhermes]

Also:

• Changing api.get() back to accept only keys and
  returns only a list (a lot of headache for not much
  gain).
• Factored out behavior to extract shared dataset_id from
  a set of keys into _get_dataset_id_from_keys().
• Updated docstrings and other tests that rely on changed /
  removed methods.

See #518 for some context.

[coveralls]

Coverage remained the same when pulling c5a634f on dhermes:add-delete-method into 0525530 on GoogleCloudPlatform:master.

[dhermes]

@tseaver I took a stab at testing / integrating the implementation from your comment.

If you already wrote this up, feel free to close.

Also, viewing _get_dataset_id_from_keys in light of #447, we should remove dataset_id from the Batch constructor and instead rely on Batch.put(), Batch.add_auto_id_entity(), and Batch.delete() to set / check the dataset ID. (This also means Batch.begin() has to move outside of __enter__.)

[tseaver]

LGTM.

[tseaver]

Merge broken?

======================================================================
ERROR: test_delete_existing_batch (gcloud.datastore.test_api.Test_delete_function)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/home/travis/build/GoogleCloudPlatform/gcloud-python/gcloud/datastore/test_api.py", line 322, in test_delete_existing_batch
connection._deleted,
AttributeError: '_Connection' object has no attribute '_deleted'
======================================================================
ERROR: test_delete_implicit_connection (gcloud.datastore.test_api.Test_delete_function)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/home/travis/build/GoogleCloudPlatform/gcloud-python/gcloud/datastore/test_api.py", line 350, in test_delete_implicit_connection
connection._deleted,
AttributeError: '_Connection' object has no attribute '_deleted'

[dhermes]

No, merge was fine, just changes to test_batch._Connection from #524 broke my tests (rightfully so).

I fixed in 2908d6f.

[coveralls]

Coverage remained the same when pulling 2908d6f on dhermes:add-delete-method into 51c1069 on GoogleCloudPlatform:master.