Add browser/bower support

I've used timekeeper on a node project before (loved it, btw), and now I needed for testing on the browser.
So I added browser support by basically wrapping the whole lib in a function that either do `module.exports = timekeeper` or `root.timekeeper = timekeeper` (where `root` is `this` – usually `window`).

Something you'd need to do yourself is to register timekeeper in bower (I could've done that, but then I'd be the package owner):

```bash
bower register timekeeper git://github.com/vesln/timekeeper.git
```