Add isReady, signalReady to views

This is based on the Oozie discussion on CDK-385. Oozie needs a way to check that a view is ready to be processed.

For an initial implementation, only views that align with partitions will be supported. The signalReady() SPI view method (carried by Signalable?) should create a "_READY" or "_SUCCESS" file that isReady() checks for. If the view doesn't line up with a partition boundary, then isReady() should throw an UnsupportedOperationException.