Fix compilation issues with rustc 1.0.0-nightly

[erszcz]

Specifically, rustc 1.0.0-nightly (170c4399e 2015-01-14 00:41:55 +0000)

I'm rather a rust newbie, so please let me know of any issues. Specifically, I'm not the happiest about repeating the match on (color_type, pixel_width), but without that the compilation failed with:

src/lib.rs:133:41: 140:10 error: match arms have incompatible types:
 expected `(fn(collections::vec::Vec<u8>) -> PixelsByColorType {RGBA8}, _)`,
    found `(fn(collections::vec::Vec<u8>) -> PixelsByColorType {K8}, _)`

If there's a better way, I'm all ears.

[jdm]

We have a branch that compiles with the 1.0 alpha - rustup_20150109. I'd prefer to rebase these changes on top of that branch and merge them in order.

[erszcz]

Haha, too bad I haven't noticed that earlier. Still, a nice learning exercise for me. Please disregard this PR, it only touches things already fixed in rustup_20150109. Thanks.

[jdm]

Thanks for the PR, regardless :)