Implements subcommands enumeration

Invoking cargo with new "--list" option now shows list of installed subcommands. (see #270)

p.s.
The code is less functional-style than I'd like it to be. Please let me know if there are smarter ways to write them.
This should use [`std::os::split_paths`](http://doc.rust-lang.org/std/os/fn.split_paths.html)