Added ssh key management commands: listkeys, addkey, removekey

`awsbox listkeys instanceNameOrId` cats the `authorized_keys` file

`awsbox addkey nameOrId /path/to/keyfile.rsa.pub` adds each of the keys in the keyfile that is not already in the `authorized_keys` file.

`awsbox removekey nameOrId /path/to/keyfile.rsa.pub` removes each of the keys in the keyfile that is not already in the `authorized_keys` file.