BF: TextBox crash on OS X

Seems that on OS X, a .ttf file can actually be a text file that
contains paths to actual TTF files. TextBox was not handling the case of
reading in a ttf file that was not a TTF.

Now fixed.
Merge branch 'master' of git://github.com/psychopy/psychopy
Merge pull request #579 from isolver/master

BF: TextBox crash on OS X