SCHEMA-793. Implement AsyncKijiResult.

Implemented AsyncKijiResult, AsyncPagedKijiResult, AsyncMaterializedKijiResult,
AsyncKijiResultScanner, AsyncDataRequestAdapter, and associated tests.

issue: https://jira.kiji.org/browse/SCHEMA-793
Calling toBytes() on a String is dangerous (without specifying charset) because it relies on the localization settings the JVM has set at startup.  Instead, use .toBytes(Charsets.UTF_8).  That will always work.