Update to new rust syntax.

`pub extern` is now obsolete. The `pub` modifier must be specified for each individual items.
Merge pull request #22 from sebcrozet/master

Update to new rust syntax.
Remove obsolete `pub` modifier on `extern` blocks
Remove unneeded @ sigils and correct formatting to the correct Rust style
Demonstrate setting a callback in the README example
Simplify that WindowDataMap interface and make it safer

This gets rid of all the potential fail cases, and decouples the `Window` impl from `private` by removing `. get_local_data`.
Merge branch 'master' of git://github.com/bjz/glfw-rs