Added hello_world example, expanded appkit

This commit adds a Hello World program, which creates and displays a
minimalist Cocoa app with a Menu Bar, a Quit menu, and a Window.

It also wraps the Cocoa methods used in hello_world in static functions
in appkit, inside separate modules for different classes. Later on,
Cocoa methods could be implemented as traits on `id` if that turns out
to be easier.
Provide NSWindow -(void)center

This is to demonstrate moving the window in hello_world.rs as an
alternative to cascadeTopLeftFromPoint.
Switch class implementation to traits/impls.
Fixed segfaul with invoke_msg_NSPoint_NSPoint. The issue was using objc_msgSend_stret instead of objc_msgSend.
Use autorelease in hello_world
Merge pull request #31 from alexchandel/hello_world

Added hello_world example, expanded appkit