Merge pull request #21 from sebcrozet/master

Fix TLS keys and safetyness.
Add kiss3d to the list of projects using glfw-rs
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`.
Reduce the reliance on the tls api in the WindowDataMap impl, and improve function names
Update to work with the new runtime