Support .dmg disk images as distfiles with new use_dmg option

[Ryan Schmidt]

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.