only output status info if both the version and source are not empty

No need output incomplete or empty statuses
have rb print the status if no args are given

use `rb @` to reset to the default (like prev `rb` behavior did).
move global .ruby-version file to `"$RB_HOME_DIR"`
fix for a bug in looking up the parent .ruby-version file
rework handling when a command line version is specified

This allows command line args to take precedence over parent
.ruby-version files yet have parent .ruby-version files override
command-line args when auto activating.
Merge pull request #33 from redding/kr-handling

Some handling reworks