SCHEMA-863. Implement AsyncHBaseKijiFuture.

Implement KijiFuture, the result of any asynchronous call in the
asynchronous implementation of Kiji Schema.

issue: https://jira.kiji.org/browse/SCHEMA-863
Merge pull request #214 from GabeBenjamin/SCHEMA-863

SCHEMA-863. Implement AsyncHBaseKijiFuture.