Rough attempt to run time trial for the code in the background (see #139)

[jlfwong]

DO NOT MERGE

This is my work trying to catch infinite loops from blocking the UI thread whenever Web Workers are available.

There are a few blockers that prevent this from being really viable to merge at the moment.

The major problem is that this is currently only working with Output.exec because Output.runCode assumes exec is synchronous - the introduction of time trial testing with a background thread makes it async.

I'm also not totally clear how the injection process with fnCalls.push works - so if I do use runCode, then the second timeTrial won't work, because fnCalls is not defined in the worker thread.

Only using exec instead of runCode makes a lot of things not work as they should in the editor (which is why runCode was introduced in the first place, I'm guessing).

[jeresig]

Moved over here: http://phabricator.khanacademy.org/D105