feat(Scope): add `$watchGroup` method for observing a set of expressions

Given an array of expressions, if any one expression changes then the listener function fires
with an arrays of old and new values.

$scope.watchGroup([expression1, expression2, expression3], function(newVals, oldVals) {
 // newVals and oldVals are arrays of values corresponding to expression1..3
 ...
});

Port of angular/angular.dart@a3c31ce1dddb4423faa316cb144568f3fc28b1a9