Converted ElasticSearchOutput MessageFormatters to Encoder plugins.
Fix test failure related to test machine time zone, and fix error in the
intro paragraphs to "Extending Heka" docs.
No longer include _timestamp key in the BulkAPI index command.
Implement code review feedback: Renamed encoders, added
ESJsonEncoder tests, fixed inconsistencies exposed by said tests, doc
tweaks.
Merge pull request #877 from mozilla-services/es-output-encoders

Converted ElasticSearchOutput MessageFormatters to Encoder plugins.