Added InterpolateUpdateService for easy updating of the interpolation symbols in templates

I'm currently working on a Django project and was having some issues with the string interpolation since I chance the default symbols to not collide with Django's template symbols.  I've added a new service to do this.   This service can be used like so (after setting up your custom interpolation symbols):

        angular.module("ngGrid").run(["$InterpolateUpdateService", function($InterpolateUpdateService){
            $InterpolateUpdateService.changeGridInterpolate();
        }]);

This is my first Angular project, so please review this commit carefully if you are considering accepting it.  I haven't gotten a chance to write tests for it yet, but I am willing to do so, if you think this is a good patch.