Re-exported OwnedBytes

OwnedBytes are needed to create a ShaderSource at runtime.
Update for https://github.com/rust-lang/rust/pull/16453
Merge pull request #320 from gaudecker/master

Re-exported OwnedBytes
use as_slice
Merge pull request #318 from csherratt/fix_master

Update for https://github.com/rust-lang/rust/pull/16453
Added generic parameter for CommandBuffer

Closes https://github.com/gfx-rs/gfx-rs/issues/321