Update path to libpng

The libpng FTP paths keep moving around--I'm pretty sure I've had to change this once before. You can't use the src/history path with the latest stable version, though, since it's only on src/. Very annoying layout.
Prepare Cairo for new BinDeps
Merge branch 'master' of git://github.com/JuliaLang/Cairo.jl into jb/working
a few cleanups