Rename gl-init references to glutin

In my attempt to build `gfx-rs` for the first time, I hit an error in referencing `gl-init-rs`. From tomaka/gl-init-rs#42, we can see the Cargo crate for `gl-init-rs` was renamed to `glutin`, causing the cargo failure.

I've renamed the folders/references in this PR, but I'm a rust n00b so I may have done something wrong :)

_Of note, I can't actually test the gl-init example on my computer since it won't execute gl-init-rs on OS X (stated in its readme), so probably wise to test build/run on your own computer. But I can confirm it does compile!_