Fixes chetan51/nupic.cerebro2#41. Moves view-specific dat.GUI control implementation to subclasses.
Addresses chetan51/nupic.cerebro2#41. Moved some dat.GUI controls back to the abstract class.
Removed "return null" from default method per feedback.
Merge pull request #43 from jefffohl/issue41

Fixes chetan51/nupic.cerebro2#41