BF: rating scale: handle skip response properly in all circumstances

- was appending a non-"None" rating in some cases; skip was still evident in the response history
Merge pull request #681 from jeremygray/master

BF: rating scale: handle skip response properly in all circumstances
Merge pull request #682 from isolver/master

ADD: iohub keyboard tests starting point
Merge branch 'master' of git://github.com/psychopy/psychopy