Second implementation of rsfml with both rc / borrow to manages resource

Again, only CircleShape is implemented to provide both rc / borrow for resource management.
Both implementation lives inside `rsfml::graphics::circle_shape`
the first one `rsfml::graphics::circle_shape::RcCircleShape`, using internally the ref counter,
the second one `rsfml::graphics::circle_shape::CircleShape`, using internally borrow pointer.

Both are reexported on `rsfml::graphics`.
So you can access them as with the current version of rsfml : 
```Rust
use rsfml::graphics::{RcCircleShape, CircleShape};
```

I'm not really fan of this implementation, i don't like to have different name for the same thing. I prefer the first way who use inner module, so you can import the module that you want.

Any opinion about this?