CommonJS compliance

Hi!

This very few additions make the code compliant with CommonJS module specifications.

It allows `angular-timer` to be:
* imported as a `npm` dependency
* used with `browserify`

See [this example](https://github.com/ArnaudRinquin/gulp-starter/blob/master/package.json#L46) on my angular + browserify + gulp seed (forked from [this awesome one](https://github.com/greypants/gulp-starter)).