Use Angular $parse service to get named values from $scope

Use of $scope[name] works only if name is simply a property identifier (denoting an object in $scope).
It does not work if the bound object is a deeper sub-object of $scope, like $scope.foo[42].bar

Newtrin0 already fixed the assignment to the named object.
This commit now corrects also the reading of the named object, fixing an error message triggered by updates sent from Firebase.
This should finally allow for binding objects deep in the scope.