Add an ssh-key command that allows you to set your public key on opsworks

closes #4 

to QA this you can run a few different attempts at this:

```
bin/opsicle ssh-key some-thing-that-isnt-a-file
# should error because its not a file
bin/opsicle ssh-key bin/opsicle
# should error because the opsicle bin isn't an ssh-key
bin/opsicle ssh-key ~/.ssh/id_rsa
# should error because thats your private key and you don't want that anywhere but on your machine
bin/opsicle ssh-key ~/.ssh/id_rsa.pub
# should actually update on opsworks YAY!
```