Don't need HiRes time now that git does VCS for us.

Also added mksymlink() utility function.  This is just
a wrapper around the core symlink function that dies
on failure. (Maybe just use autodie instead?).
Add constants for validating usernames.
Rename current_user() -> current_username().

Also added a Type for usernames that is resticted to just
alphanumeric types.
Change current_user() -> current_username() in tests too.
Bring over tests that were still in the Pinto core.
Add tests for parse_dist_path.