Add SSH key upload feature

[guyzmo]

add a command to upload SSH keys to the service, something like:

git <target> ssh push <ssh-public-key-file>
git <target> ssh ls
git <target> ssh delete <ssh-public-key-id>

the whole point being to make it easy to generate a new key (or even rotate keys!) or revoke an old key with the CLI.

[guyzmo]

(needs to have tests written)