Add a ZK shell

This is pretty much a handy clone of zkCli.sh that I use for the
same things (but I prefer to use a Kazoo based one - since it's
what the clients I deal with are using).

It supports the basic ops:

python2.7 contrib/shell.py localhost:2181
(CONNECTED) /> ls
zookeeper
(CONNECTED) /> create foo 'bar'
(CONNECTED) /> get foo
bar
(CONNECTED) /> cd foo
(CONNECTED) /foo> create ish 'barish'
(CONNECTED) /foo> cd ..
(CONNECTED) /> ls foo
ish
(CONNECTED) /> rmr foo
(CONNECTED) />

readline support is enabled (if readline is available).