Memory leaks

[lordmauve]

I've been investigating some serious memory leaks crashing one of my apps, and have traced these back to Throbber.js.

It looks like there are two reasons behind the memory leak:

1. the animation loop runs forever, even when a throbber is stopped/removed.
2. the draw() function calls ctx.save() without calling ctx.restore(); this will effectively push more and more context snapshots onto the stack.