Implemented mkdir

Feel free to code review, made some changes to the makefile as well. Not sure exactly what the --test option in rustc does, but whenever I ran it it just spat out dead_code warnings which weren't true, so I removed it.

Also the cat util's test was out of date with the rust branch so I updated it. I've been trying to find out how to use #[test] lately for unit testing but I can't seem to find any updated documentation on it which is a bit frustrating.

Also, this was only tested on Linux.