Key revocation list command missing

[edoput]

We can currently sign ssh keys but are unable to revoke them.

Certificates can be revoked with one of two inputs:

• by key ID
• by certificate serial

the command is ssh-keygen -s /path/to/ca.pub -k create a KRL, the command ssh-keygen -s /paht/to/ca.pub -u update a previous KRL

[cimbalo]

Depends on #10 (We are actually unable to retrive the ca who issued a certificate).