Add list as base method and allowed_methods decorator

Hey, me again:

I added list() to the ModelEndpoint base class so that it is now automatically available at:

/ajax/{app}/{model}/list.json

I also added a decorator called allowed_methods which can be used to limit the methods that can be accessed on a ModelEndpoint. 

The idea is that you decorate your ModelEndpoint's authenticate model with it. 

Something like:

class CategoryEndpoint(ajax.endpoints.ModelEndpoint):

    @allowed_methods(['get','list'])
    def authenticate(self, request, application, method):        
        return True

        ...

I also updated the README to reflect these changes.

As before .. feel free to use or discard as you like :)