add 'KeyList' method to datastore

[whyrusleeping]

Let me know how this looks. I didnt implement the one for elastic because i couldnt build it on arm.

[jbenet]

I'm ok with doing KeyList for now, but in long run will want to use a Query func, like the python version:

• http://datastore.readthedocs.org/en/latest/api/query.html
• https://github.com/datastore/datastore/blob/master/datastore/core/basic.py#L58-L71
• https://github.com/datastore/datastore/blob/master/datastore/core/query.py

[jbenet]

awesome, thanks @whyrusleeping !

[jbenet]

awesome, thanks!