fix(directive/select/ngOptions): ignore properties which start with $ or $$ when iterating over an object for ng-options

When using ```<select ng-options ...``` with an object, the directive renders properties of the object even if they begin with ```$``` or ```$$```. This means that when using ```$resource``` to ```query()``` a REST service for example, the directive renders the private ```$then``` and ```$resolved``` properties of the response object as ```<option>```s. The ```ng-repeat``` directive includes a check to make sure it doesn't render properties beginning with ```$``` or ```$$```, and this PR adds a similar check for ```ng-options```.