Allow get() to take a single key (in addition to a list)

[dhermes]

Has #520 as diffbase.

[coveralls]

Coverage remained the same when pulling 923302a on dhermes:allow-get-single-key into 77cfe10 on GoogleCloudPlatform:master.

[tseaver]

I've avoided collections.Iterable in the past, because it matches str / unicode. Maybe we don't care about that here.

[tseaver]

Other than that, LGTM

[dhermes]

My take is that if the iterable does not contain Keys then it will fail at the point of keys[0].dataset_id.

For example I accidentally left [key] in Test_get_function._miss_helper

        if use_list:
            key = [key]
        results = self._callFUT([key], connection=connection)

and got an error like

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'list' object has no attribute 'dataset_id'

[coveralls]

Changes Unknown when pulling 184cfb9 on dhermes:allow-get-single-key into * on GoogleCloudPlatform:master*.