Configure SubType via object with more customizations

[ianwremmel]

Still needs some tests, but much more robust than #4.

1. extend() now accepts an options object as alternative to the subTypeName and errorCode parameters.

2. Decreases emphasis on HTTP-related errors. (instead of passing errorCode, add {properties: {code: <code>}} to the options object.

3. Adds ability to provide alternate toString().

4. Adds parseFn(), which can do additional processing on the SubType constructor argument. I'm primarily envisioning it as a way to wrap http error responses with an Error:

   if (res.statusCode === 404) {
     throw new HttpNotFoundError(res);
   }
   

5. Just like #4, makes monkeypatching optional.