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 may want to or-in the execute bits instead of just setting the bits, I've seen some weird failures on windows in the past with odd permissions