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
Looking a little deeper I'm guessing now that this is because you want to return a Future<Object> instead of Future<List<Object>>. If that's the case then I think this is fine, or you can just always create a new object which simplifies the implementation at the cost of a tiny bit of runtime. You should probably also add a note here about the purpose of the callback.