refactor(ngAria): DRY camelcase method

@petebacondarwin I'm using attr.$normalize as you suggested in https://github.com/angular/angular.js/pull/10337#issuecomment-65783835

The problem is that we also need to [normalize the name in the service](https://github.com/cironunes/angular.js/blob/325f609582992730f43d2ccf859bbc9f6d03e6c1/src/ngAria/aria.js#L181). Probably that's why the local ```camelCase``` method was created in the first place.

I still think that we should have a public ```camelcase```as we do with ```lowercase``` and ```uppercase```, but I'm not sure how to better approach this, what do you think?