Removing datastore.set_defaults() from all docs.

[dhermes]

Fixes #685.

NOTE: I did not remove the actual method from datastore.__init__ but easily could.

Here are the remaining uses of it.

In datastore:

$ git grep set_defaults 
gcloud/datastore/__init__.py:def set_defaults(dataset_id=None, connection=None):
gcloud/datastore/test___init__.py:class Test_set_defaults(unittest2.TestCase):
gcloud/datastore/test___init__.py:        from gcloud.datastore import set_defaults
gcloud/datastore/test___init__.py:        return set_defaults(dataset_id=dataset_id, connection=connection)

And storage:

gcloud/storage/__init__.py:def set_defaults(bucket=None, project=None, connection=None):
gcloud/storage/test___init__.py:class Test_set_defaults(unittest2.TestCase):
gcloud/storage/test___init__.py:        from gcloud.storage import set_defaults
gcloud/storage/test___init__.py:        return set_defaults(bucket=bucket, project=project,
regression/storage.py:storage.set_defaults()

[tseaver]

LGTM

[dhermes]

@tseaver Sorry I just pushed another commit onto this also fixing #684.

I'm going to merge it as well (only a few lines and 100% in one file in one docstring). Let me know if there are issues.