let client request an on-demand probe

[danwent]

if a client saw something other than what the notary reported, let the client request a new "on-demand" probe.

we should do something simple to rate limit the number of on-demand probes allowed, since a client could otherwise use a notary to 'hide' its identify while flooding requests to another site.

[daveschaefer]

This is probably a duplicate of #20