ENH: More work on TextBox

- (hopefully) fixed libfreetype loading issue on OSX.
- Added exception handling if invalid font file is read
- Updated examples to select an available font to use, so demos should
run on all 3 OS's without change (they do for me atleast)
- Huge improvement in time taken to create a TextBox stim.
- Misc. fixes and updates.

- Still in progress.