Add a `main` field to package.json

Hey @urish it's Elfrey [again](https://github.com/urish/angular-spinner/issues/5).

We're still actively using `angular-spinner` with [browserify](http://browserify.org/). However, without a `main` field in the `package.json`, we have to do some ghetto workarounds to use it with browserify.

It'd be nice to have it to enrich the quality of our lives. :earth_africa:

Afterwards, make sure to publish the changes to `npm`.