Add writing support to Engine/Workers

[thiell]

This ticket follows the feature of '''writing''' to Engine clients and associated workers, allowing write() on workers like WorkerSsh.

This is a necessary feature for ClusterShell v2 to send live commands to distant ssh's. Also, the [wiki:clush] tool will be improved by allowing things like:
{{{

echo "foo" | clush -w mycluster[1-32] cat

}}}
(this is not supported by pdsh ;-)

[thiell]

[st-cea] Library support committed in revision 92. Need to rewrite a bit the clush script to support the example in this ticket.