TESTS: several adjustments
TESTS: adjust travis install for pyserial pycrsltd
ENH+DOC: maskParams: allow user to just specify one parameter in dict.
DOC: added doc on behavior of GratingStim.sf = None
Merge pull request #608 from jeremygray/master

test tweaks and BF for rating scale component
Merge branch 'master' of git://github.com/psychopy/psychopy into attributeSetters
BF: 1. texRes did not apply to tex.
DOC: texRes has to be power-of-two.
BF: do not log calls to mask and tex when setting texRes.