New rust-sfml implementation

I've made all inner modules from graphics / sounds etc private, so the we can't anymore access module from the inner module.

`Borrow` implementation are made the default implementation.
You can access one implementation or the other for a shape like this : 
```
use rsfml::graphics::CircleShape       \\ Use borrow to manage resources
use rsfml::graphics::rc::CircleShape   \\ Use rc to manage resources
```
I've make one limits to this, in the case of you draw with `RenderStates`. 
For example, if your shape use `borrow`, you can't draw with a `RenderStates` using `rc`, and vice versa.