Implement md5sum (closes #47)

To get this to work, I had to add @DaGenix's [```rust-crypto```](https://github.com/DaGenix/rust-crypto) as a submodule.  This required some fairly significant changes to the Makefile, mainly revolving around adding the ability to execute custom Makefiles for the utilities that need them (see ```md5sum``` for an example).

This is also fixes ```truncate``` for the latest Rust.