Added Ember.getProperties

`getProperties` has always existed on `Ember.Object`'s prototype, but
it's been unavailable as a utility function to use on POJO's.
Now `Ember.getProperties` can be used with non `Ember.Object`s, e.g.

    var obj = { foo: 5, bar: 6, baz: 7 };
    Ember.getProperties(obj, 'foo', 'bar'); // => { foo: 5, bar: 6 }
    // equivalent to
    Ember.getProperties(obj, ['foo', 'bar']); // => { foo: 5, bar: 6 }