Add multiple public keys #23
Description
Currently the user model has a 'public_key' field, which holds a string of the user's only key. However, many users will want to set multiple public keys which they can use from many different machines. We should be able to support having multiple public keys available.
The model implementation is simple: Create a 'public_key' model with a user_id and link to the user model as many_to_one (and vice-versa).
The public key is used in one place, User#valid_signature? This method also needs to support verifying a signature against multiple public keys.