Implement parts of lua_gc

[daurnimator]

Collection:

• IE has window.CollectGarbage
• If node is started with --expose-gc then you get a global.gc function.
• Chrome can be started with a similar flag to get window.gc
• Opera has window.opera.collect()

Accounting:

• Webkit browsers have performance.memory.usedJSHeapSize

[giann]

This could demand a lot of work. Maybe the payoff is not worth it.

[daurnimator]

We can go arg-by-arg, browser-by-browser. We'll figure it out as we go.