ENH: visual.RatingScale better marker='hover'

now the marker changes in response to moving off the labels and back on
ENH: more efficient hover in RatingScale
ENH: another small efficiency in RatingScale

also helps the readability of the code
Merge pull request #505 from jeremygray/master

ENH: visual.RatingScale better marker='hover'
Merge pull request #507 from peircej/master

fixed fitCumNorm test and fixed recent ElementArrayStim rendering glitch
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
BF: fix failed string splitting for PR650

Presumably we used to import the string module, but this shouldn't be
necessary. (And Jon hasn't had a PR650 for a while so the code has gone
untested)