Minor documentation corrections for load_ssh_public_key
Removed redundant tests for _load_ssh_rsa_public_key since it is now a non-public part of the API and made a number of minor changes to tests and documentation
Added better parsing for RFC 4251 string and mpint values.
Also moved several of the SSH key splitting and validation checks up into the load_ssh_public_key method since they will apply to more than just RSA.
Added additional checks to make sure the key doesn't contain extraneous data