Allow manual flushing of a batcher with flushBatch method

Implements part of a TODO item listed in comments in BatchExecutor.scala

Future.batched now returns an instance of class Batcher, which has the added method flushBatch to allow manual flushing/execution of remaining items in the batcher.
Although narrowing a return type is kosher for source compatibility, it is unkosher for [binary compatibility][0].  We should probably either add another endpoint, or punt on this until we decide to do a major version bump.

[0]: http://docs.oracle.com/javase/specs/jls/se7/html/jls-13.html