Map resource

Adds a `map_resource()` function that takes the URL of an endpoint as the first arg, the resource class as the second, and passes along `**kwargs`.

Resolves #299. 
This code is duplicate of `add_resource`. I know thats kindof intentional, but it would be much simpler to just call `add_resource` with the arguments switched around. That way the two ways of adding resources wouldn't get out of sync