Wrap pebble objects to add lifetime support / safety

- Create a PebbleObject type which has Create and Destroy methods, as well as GetRootObject (or something like that) and implements the Drop trait (calls the destroy function on drop). (see http://doc.rust-lang.org/core/ops/trait.Drop.html)
- Subclass windows, ... etc. from this trait
- lib functions now need to unwrap the C objects from the rust ones before they pass it to the c functions.
