Merge pull request #42 from servo/enum

Force checking the color type before accessing pixels
Cargoify rust-png
Merge pull request #43 from metajack/cargoify

Cargoify rust-png
Fix the branch used for Cargo
Merge pull request #44 from metajack/cargo-branch

Fix the branch used for Cargo
Decode grayscale images to RGBA

Part of the fix for servo/servo#3154.