fix(btnRadio): set default value if used inside repeater

If using ngRepeater to create a set of btnRadios, default value wasn't setted properly. Based on [angular doc](http://docs.angularjs.org/guide/directive#attributes):

> Use $observe to observe the value changes of attributes that contain interpolation (e.g. src="{{bar}}"). Not only is this very efficient but it's also the only way to easily get the actual value because during the linking phase the interpolation hasn't been evaluated yet and so the value is at this time set to undefined.

Here is a plnkr with an [example](http://plnkr.co/edit/DzZZ3VCHRZ80li9ZsW3f?p=preview). It doesn't show as active the first button even if it is marked as selected.