Features/elasticsearch output

This merges dev branch (which is where active Heka development happens) into features/elasticsearch-output, and makes a few small tweaks to elasticsearch_output.go.

If you'd rather keep the branch merge separate from my tweaks, then you can just merge dev (make sure it's updated from mozilla-services/heka!) into features/elasticsearch-output yourself. If git cooperates, that should reduce this pull request to just my changes.