JWS API Changes

A couple of alterations to the JWS API to account for strings to be signed as opposed to only JSON objects.

If you pass jws.sign() a string, it will no longer try to parse it as a JSON object. Additionally, jws.verify() now returns a string instead of attempting to load that string as a JSON object.

Strings are perfectly valid JWS payloads.

The extra step of loading the JSON object has been moved to jwt.decode() because that is required to be a JSON object.

RSA JWKs

A JWK dictionary can now be passed directly to RSAKey.prepare_key() in order to construct that key. Once this works for every key type, I am going to bump to version 1.0.