Add a WebAPIResource class for creating more RESTful APIs.

This introduces a new class for easily creating RESTful URL resources
that can handle standard HTTP methods and return meaningful data in a
nicely constructed hierarchy.

By using WebAPIResource, a site can offer a very clean set of URLs to
work with any object.

There's a lot to this object, but it's all covered in the extensive
documentation in webapi/resources.py.

Review Board will be using this for its new more RESTful API.

There's more I want to do for this soon, such as automatic documentation
generation and proper mimetype detection, but I wanted to get this in
as a base first.

Tested this with my upcoming Review Board change for the web API.