Added RustyXML as submodule

I want to parse the CraigsList XML feed, so I'm
adding this library.

It turns out that you have to pass the rust compiler
the -L flag for library, so it knows where to find
your libraries.

Also, the RustyXML library has to be compiled into
a crate (.rlib file) before iron-santa can link
that library into the final executable.

Therefore, I've significantly rewritten the Makefile
to reflect the rustc way.
Adding submodule remote reference