Possible implementation for use both rc / borrow to manage resources.

Only an implementation for the CircleShape is made for the moment.

each implementation have move on a submodule : 
`rsfml::graphics::circle_shape::CircleShape` become 
* `rsfml::graphics::circle_shape::rc::CircleShape`
* `rsfml::graphics::circle_shape::borrow::CircleShape`

both are reexport in the module graphics. So you can use them like this:
* `rsfml::graphics::rc::CircleShape`
* `rsfml::graphics:::borrow::CircleShape`

You can use both implementation in the same program.
So at least you should be able to do this kind of import : 
`use rsfml::graphics::borrow::*;` and
`use rsfml::graphics::borrow::{CircleShape, RectangleShape};`
or
`use rsfml::graphics::rc::*` and
`use rsfml::graphics::rc::{CircleShape, RectangleShape};`
to use all the shape of on implementation or the other.

or both in the same program
```
use rsfml::graphics::borrow::CircleShape;
use rsfml::graphics::rc::RectangleShape;
```

What do you thing about this design?

@ShortStomp?