signer works again with string public keys

See https://github.com/namshi/jose/issues/15#issuecomment-67822189
hey @eroux, thanks for caring! :)

I would go for a different approach and use [openssl_pkey_get_public(...)](http://php.net/manual/en/function.openssl-pkey-get-public.php), so something like:

```
if (!is_resource($key)) {
  $key = openssl_pkey_get_public($key);
}
```

What do you think? Would you also be able to add a test?