Add response collection option

Record RT since the appearance of the sequence item.
change default option for choices parameter
PreloadImages method will auto-flatten the input array

Closes #42
Remove normalize trial variables method

This is a bad idea for this plugin, since check_fn is a function. Need to figure out a way to protect variables like this one. For now, just revert to no functions as trial variables for this plugin.
Allow for protected variables in normalizeTrialVariables

Related to #43
protect check_fn in normalizeTrialVariables
make sure unprocessed functions get preserved in trial obj

related to issue #42
removing dev folder from master branch. see branch plugin-dev-old for recovery.
remove variables option

issue #44
Merge branch 'master' into smart-arrays

Conflicts:
	jspsych.js
delete files - from master merge