TMI: First rustpkg pull request!

So I finally submitted a pull request with my initial work on rustpkg. I'm embarrassed at how little this patch actually does; I've tested it with one command, build, on one package, my fork of rust-sdl. The difference now is that as per #5679, rustpkg doesn't require a package to explicitly declare its ID and version number; instead, it just looks for a directory with the name you give it. So if you write rustpkg build rust-sdl, rustpkg will expect the current directory to contain a directory called rust-sdl that contains the package. Likewise, if it was rustpkg build rust-sdl-0.1, it would infer the version to be 0.1 (though I don't think I actually implemented that part yet).

I still have to figure out how to write unit tests for this. Also, rather than trying to do everything at once, I made several of the commands other than build fail with "not yet implemented". rustpkg wasn't in a great state anyway, so I don't think this will make anyone too unhappy. I think it's better to explicitly fail than quietly do something not-quite-right.

Hopefully I can proceed faster now that I'm past the awkward initial "what am I doing?" stage of the project.