jspsych-similarity.js : fixed data storage so that generic data object
was included in the block's data.
jspsych-storybook.js: included generic data object in data storage,
included path to image in data storage.
jspsych.js: cosmetic adjustment to code in finishExperiment() method.
NEW:
added call-function plugin, which calls a user-specified function.
useful for saving data before completion.
added paint plugin - displays a canvas element superimposed on an image
and allows subjects to draw on the image to indicate areas of interest.
