Add `mog' command-line tool as an example/frontend

Add `mog' command-line tool as an example/frontend

This supports several UNIX-like subcommands:

cp FILE KEY       - copy a file to a given key

cat KEY(s)        - cat any number of keys to STDOUT

ls PREFIX         - list keys matching PREFIX (not globbing)

rm KEY(s)         - remove keys

mv FROMKEY TOKEY  - rename a key

stat KEY(s)       - show various information, including URLs and Size

tee KEY           - read input from STDIN and write it to key
                    (due to the limitations of HTTP servers
                    and clients this is not streamed)