KIJIMR-14. Gatherer that writes to a Kiji table.

 - Adjusts Job builders:
   - Gatherer job builder customized for HFile outputs.
   - Gatherer has a withConf() to inject a base configuration.
 - Various examples for integration tests.
 - Consolidate configuration keys in KijiConfKeys. Better naming consistency.
 - Update job builder unit-tests.
 - Several integration tests for gatherers.
 - Update JobOutputSpec:
   - location no longer optional.
   - KijiTableMapReduceJobOutput becomes DirectKijiTableMapReduceJobOutput
   - hfile now parses as "hfile:kiji://hbase/kiji/table;hdfs://path/to/hfile@split"
 - Updated job builder unit-tests to use fake HBase instead of mocks.