Change DatasetBatchStage constructor to follow generic Stage constructor interface.

DatasetBatchStage now inherits from Stage.
Remove unused source method on DatasetBatchStage.
Merging dataset batch stage implementation and start pipeline stage removal.
Completely Remove PipelineStage.

All the responsibilities are now handled by Stage. Validation of the
sinks for a stage has currently been ommitted as a distraction.
Pull DatasetBuilder out into its own file.
Cleanup, and remove some dead code.
More specs.
Version bump to 0.1.4
Regenerate gemspec for version 0.1.4