Improve implementation of create_shared and turn it into a method
Improve implementation of create_shared and turn it into a method

Window::create_shared is now a method on &self. Call it to break off a new shared window.

Internally Window::{create, create_shared} both use a new function Window::create intern that takes an Option<&Window> for the shared context. This means that Window::create does not need to make an empty window for the supplied shared window parameter.

jhasse/glfw-rs,erickt/glfw-rs,nstoddard/glfw-rs,PistonDevelopers/glfw-rs,mneumann/glfw-rs,servo/glfw-rs,larsbergstrom/glfw-rs,derekdreery/glfw-rs,tomaka/glfw-rs,bvssvni/glfw-rs,derekdreery/glfw-rs,bjz/glfw-rs,brson/glfw-rs,rlane/glfw-rs