Rename "osmain" to "compositing"
Separate out windowing from compositing
servo: Remove the type parameter from the compositor, rename OSMain, and refactor engine a bit
servo: Warning police.
Split the network stuff out of servo-gfx into servo-net
Add missing compositing/mod.rs
servo: Address review comments
Add in some missing dependencies.

Fixes #423.
Remove warnings from servo-gfx
Remove unused code