NF: class visual.InstructionScreen

goal = simplify code in coder demos
design pattern = display text, wait for any key, quit on escape
RF: a couple code simplifications, esp rating scale demo
RF: better visual.InstructionScreen

simpler, and kwargs can now control the format of the TextStim
RF: add InstructionScreen() .show() method

- allows separation of instr object creation from its display / use
DOC: fORP when using USB
DOC (?): use "default text" as default text; closes #462

previously was "nonsense"
REVERT remove visual.InstructionScreen
Merge pull request #453 from jeremygray/master

some minor code simplifications