Rename sshkey-add to key-add. Add keys and key-remove commands

Usage examples should be self-explanatory:

```
$ hk keys
5e:67:40:b6:79:db:56:47:cd:3a:a7:65:ab:ed:12:34  user@test.com

$ hk key-remove 5e:67:40:b6:79:db:56:47:cd:3a:a7:65:ab:ed:12:34
Key 5e:67:40:b6:79:db… removed.
```