Support recursive handler loading, additional methods

- switch to a Swagger 2 based handler spec. See
  https://github.com/gwicke/restbase/blob/master/doc/Architecture.md#declarative-proxy-handler-definition
  for an example.
- rename fromHandlers to loadHandlers, and let it recurse into subdirs that
  aren't complete modules
- add a new fromHandlers method which lets users pass in a separately required
  module implementing the Swagger handler spec