Add ability to "push" new items onto the list/promise returned by getList()

Not only is it a timesaver, but it saves a whole section from the README file :)

e.g.

````javascript
    $scope.cats = Restangular.all("cats").getList(); 
    // Sometime later
    $scope.cats.push({ name: "Mister Tinkle" });
    // Magically appears in the view along with the rest of the data!
````