Fixed an issue where globals weren't being picked up

I noticed that when I was using this module the global options I had set up in my `.jshintrc` file were not being picked up by `jshint`. According to [this comment](https://github.com/jshint/jshint/issues/710#issuecomment-15539466) the globals need to be passed in as the third parameter when calling `jshint`, so I've amended the `index.js` to reflect this.

Note that `jshint` seemed to work better when the globals were removed from `.jshintrc` than if they were left in which is why line 53 is there (otherwise I would have left it out).