ENH: tweak keyboard component deault duration

- typically want keypress to end routine, not last for 1s, so set default duration = empty
BF: KeyComp: 'escape' works with empty Allowed Keys; closes #467

- fix case in which empty -> any key -> slurp up 'escape' -> not detected

Notes:
- arbitrary events could escape/quit very easily, just set endExpNow = True
FF: escape now works even if no keyboard component

also minor tweaks to keyboard.py
RF: remove redundant 'escape' from ratingscale component

should be handled by outer loop
Merge pull request #470 from jeremygray/master

Keyboard component 'escape', default duration