DOC: typos in callOnFlip doc string
ENH: add RatingScale history (for continuous ratings)

- automatically records a list of (current rating, time-stamp) tuples
- enables continuous ratings to be obtained (by recording all intermediate values)
- usage illustrated in coder demo

*Changes* how RatingScale.getRating() works: no longer returns False prior to an accepted rating
  (now returns the currently selected value, even if that's not the final value)
ENH: tweak to RatingScale history

- the last history item should include the RT when accept was pressed or timed-out
Merge pull request #260 from jeremygray/master

DOC typo; RatingScale.history