More work towards a sane ruby binding

### The Things

 * [x] Moved to `CSG::Native`
 * [x] Bound `stl_free` and `free_bsp_tree`
 * [x] Add finalizers
 * [ ] ~~Wrapper class as `CSG::Solid` with ruby-like interface~~

cc/ @sbryant 

**Targets: #24**