When using @requires the requires method is auto-generated. However, as
it just takes a list the tasks run method needs to identify which input
is which.

This adds support for named requirements using luigis existing support
for returning a dictionary from the requires function.

Usage:

    class Parent1(luigi.Task):
        ...
    class Parent2(luigi.Task):
        ...

    @requires(first_parent=Parent1, second_parent=Parent2)
    class Child(luigi.Task):
        def run(self):
           first_parent_target = self.input()["first_parent"]
           second_parent_target = self.input()["second_parent"]