Save trialdata to csv format

This is one potential way to fix #79. It actually preserves the
user-saved data on each trial as a JSON string within the csv file (so
each row in the csv is [uniqueid, current_trial, dateTime, trialdata]).
Merge branch '79-save-trialdata' of https://github.com/jhamrick/psiTurk into jhamrick-79-save-trialdata