Add an exception handler to JavaScriptCore.

It currently just NSLogs the output.
Add app.js for all the OSX app related javascript.
Rename set_result_text to setResultText.
Add an NSTextField 'timerText'.
Add 'setTimerText' as a Javascript API.
Implement 'setInterval', 'clearInterval' and 'getMilli'.

Currently this uses a single NSTimer, so won't work if there is more than
one user of setInterval.