FF: ratingscale key handling
ENH: add autoDraw to class RatingScale

inheriting from BaseVisualStim still does not seem right, so just copy & paste a method here and there
DOC: RatingScale autoDraw doc string
RF: split BaseVisualStim into three classes

All test_all_visual/ tests are passing. New internal hierarchical structure:
- non-visual housingkeeping methods (MinimalStim) = the core of the core
- deprecated visual methods (LegacyBaseVisualStim)
- current visual methods (BaseVisualStim)

RatingScale benefits if it can inherit from MinimalStim, reducing code redundancy, look for another commit
RF: visual.RatingScale now inherits from MinimalStim

removes a lot of code redundancy while avoiding unnecessary visual methods in a RatingScale (if we were to just inherit from BaseVisualStim)
Merge pull request #510 from jeremygray/master

implement autoDraw for class RatingScale