SCHEMA-886. Implement asynchronous writing using AsyncHBase.

Implemented and tested AsyncHBaseAsyncKijiBufferedWriter to
support asynchronous writing to a Kiji table.

issue: https://jira.kiji.org/browse/SCHEMA-886
The objects returned by this callback don't have any particular meaning, right? They're just here because you need type parameter for the Deferred and you can't instantiate Void. If that's write, then what is the purpose of this callback at all? Is there any significance to the fact that you're returning the last element of this list instead of the first, or any random element?