TEST improved testing of ratingscale.py

- 97% coverage, more asserts, use TEST_FONT
- minor tweaks to the ratingscale code to avoid bad values
Merge pull request #389 from jeremygray/master

TEST improved testing of ratingscale.py
TEST visual.helpers: contains(), .overlaps()

- 100% coverage of contains() overlaps() if you test using matplotlib 1.2+
Merge pull request #390 from jeremygray/master

TEST visual.helpers: contains(), .overlaps()