cnclient: provide email proof

[andres-erbsen]

https://github.com/yahoo/coname/blob/master/cnclient/client.go#L107 should provide a DKIMProof on initial registration. This would be an email sent from the address being registered to the address being registered with the subject line containing the prefix specified in server conf and then the hash of the entry. Lack of this proof will likely cause an io.EOF on the server side prior to 5fe00a3.