simple_key_value_store input validation lacking

[Fraser999]

If the following invalid input is entered to the simple_key_value_store example it results in a crash:

get
put
put 1

If put 1 1 1 is attempted, it is treated as put 1 1.

I highly recommend (ab)using docopt for this in the same way as has been done for the crust_peer example which initially suffered from identical problems:
https://github.com/Fraser999/crust/blob/master/examples/crust_peer.rs#L77-L92
https://github.com/Fraser999/crust/blob/master/examples/crust_peer.rs#L432-L504