Support .dmg disk images as distfiles with new use_dmg option

I'd like MacPorts to support distfiles in .dmg disk image format, in addition to the .tar.gz, .tar.bz2 and .zip formats currently supported. I spent some time working on this tonight and came up with an implementation that works for me. It's my first bit of hacking in MacPorts base, though, so I'd appreciate any and all feedback, both on the idea and the implementation. Here's the ticket, which includes the patch: http://trac.macports.org/projects/macports/ticket/13509 Thanks.