Allow methods to be included in response

There are cases when the complexity of a method on a data model exceeds what is appropriate or possible using a hybrid property. I believe flask-restless should enable having this kind of logic on the model, by allowing return values of arbitrary methods to be included in the JSON response objects.

In this pull request, I have made the following changes:

Added an `include_methods` argument which allows embedding return values of methods into JSON response structures. Also, it serializes SQLAlchemy mapped objects returned from an included method.