Added ngModel so you can access the input model outside the directive

This was originally pull request #5 but I had done a pull request from my `master` branch to yours which was not my intention.

I've branched the fix so that it only includes the ng-model fix. Sorry about that.

Comment from my original pull request:
> This was actually brought up in Issue #4 by @stroebele.
> 
> Hope this helps.