Add ssh public key bits method

    Moved ssh public key unpacking into private method
    Call that method for both validation and bit return
    Use new public class method for public instance method #bits
    Add testing for new method including caputuring exceptions
Add SSHKey::PublicKeyError

    Add exception class inheriting from StandardError
    Raise errors in public class methods on invalid keys
Merge pull request #10 from dacamp/master

Add public class method ssh_public_key_bits