make RenderWindow mutable in Drawable traits

Solve a legacy problem, with a non mutable render_window you can't call draw which need a mutable render_window
in the past all the reference in rsfml were not mutable, so i forgots these.
add two more examples

the first show how to share the same resources using borrow pointers,
the second show how to share the resources using reference counting
update wrong doc
replace null pointer check functions by methods
add deriving to ToStr and Show for non-ffi structs
add a new constructor to VertexArray
fix to last rust

- extern crate ...
- ptr::to_unsafe_ptr removed
add new ways to access vertex in the VertexArray

- An iterator to use with the rust for loop
- modify the get_vertex to be mutable
- add trait Index to access with v[42].

+ add exemple to show how use these accessors

Thanks to @tiernjo to show me these lacks in rsfml!
clean code (remove trailling whitespace)