gcloud.datastore.key.Key.from_path is undocumented, untested

[tseaver]

I'm not sure what its contract is, either:

>>> from gcloud.datastore.key import Key
>>> Key.from_path().path()
[{'kind': ''}]
>>> Key.from_path('abc').path()
[{'kind': ''}]
>>> Key.from_path('abc', 'def').path()
[{'kind': 'abc', 'name': 'def'}]
>>> Key.from_path('abc', 'def', 'ghi').path()
[{'kind': 'abc', 'name': 'def'}]
>>> Key.from_path('abc', 'def', 'ghi', 123).path()
[{'kind': 'abc', 'name': 'def'}, {'kind': 'ghi', 'id': 123}]

Dropping the odd last element doesn't seem correct.