always use jws.sign such that errors can be at all handled

[omsmith]

This PR switches sign to always use jws#sign instead of the streaming interface. Why?

1. I was going to open a PR to handle error events from the stream, however callback is not an errback (oops!), as such the only way to actually catch/handle errors if for them to throw synchronously - and thus we need to use the more-synchronous method.
2. The stream interface calls jws#sign anyway, after buffering input data. In this case, all inputs are already known (and small), meaning the streaming interface just adds overhead
3. Similar to 2, and my sentiment in Return a Promise #111, this is all synchronous anyway, and the callback APIs provide little-to-no value (though atleast verify is an errback!)

[jfromaniello]

You are right... But I'd rather change the next major version to do be an errback

jws.createSign({
   header: header,
   privateKey: secretOrPrivateKey,
   payload: JSON.stringify(payload)
}).once('done', function (signature) { callback(null, signature); } )
  .once('error', callback); 

does it make sense?

[jfromaniello]

I will close this in favor of handling #169 in the next major release.