allow inherited resource class to be added

[ollofx]

add_resource method of Api class searches for declared methods ['GET','POST'...] inside:
f = resource.dict.get(method, None)
dict only reflect the local class methods to be able to reflect base class method we can use
f = getattr(resource,method)
this will allow for instance to inherit a base class CustomerResource(Resource) which has property many=False into another class CustomersResource(CustomerResource) with many=True overloaded
then we could declare:
api.add_resource(CustomerResource, '/api/customer')
api.add_resource(CustomersResource, '/api/customers')

[dmytrokyrychuk]

Hi @ollofx, I was just passing by and noticed that you used the future package here. I think it's better to use six instead because Flask-RESTful already uses it, and anyone who uses this package is most likely using Flask-RESTful too, and therefore they will have six installed anyway. I think there is no need to install two libraries that provide similar features.