Pull in changes from the upstream master

Looks like a lot of improvements as well as the ability to support a few other image formats like TIFF and BMP.

This PR includes a new dependency on code.google.com/p/go.image which is an imaging package written by the core Go team but just not in the standard library.

Once this gets merged in https://github.com/DramaFever/image-boss/pull/27 will make sense.