add failing_noop operation + client command sign/check arbitrary message

implements https://gitlab.com/cryptiumlabs/tezos/issues/52 #81 (closed)

Add an Failing_noop operation with tag 7. This operation is never considered by the state machine as it always fails at apply.

This new operation was added bearing in mind the possibility for the end user to sign somethin and to be sure that it will never have a meaning.

Also this MR adds two new client commands that use this new operation :

tezos-client sign message <str> for <implicit_account>

tezos-client check that message <str> was signed by <implicit_account> to produce <signature>