SAMZA-266; allow empty checkpoint factory config
SAMZA-253: Consensus shutdown API and per-task commit API. Reviewed by Chris Riccomini.
SAMZA-269: SerdeManager should pass through envelope objects if not modified
SAMZA-264; convert Samza website to bootstrap
SAMZA-138; add a file system consumer
SAMZA-267; offset manager should not fail if task.inputs is changed
SAMZA-186: Upgrade Samza to YARN 2.4
SAMZA-271; remove dead code from kafka system consumer
Merging back to trunk.