Implement Drop for Window

This automatically cleans up TLS when the window is destroyed. We turn to the dark arts when invoking callbacks, but this shouldn't cause any leaks.
 Remove get_current_context

Unfortunately this is incompatible with the impl of Drop on Window. Creating a new Window with the same handle would cause the TLS to be cleared from under us. If you'd like to know whether a context is current, you can use the Window::is_current method implemented in the last commit.
Fix uses of Option::get in examples

Option::get copies, where as Option::unwrap consumes