Allow modules to require services

A module should be able to express that it relies on services.

Something like:

class AService(kp.Module):
    def configure(self):
        self.expose(self.say_hello, 'say_hello')

    def say_hello(self, to='unknown')
        print("Hello {}".format(to))


class AModule(kp.Module):
    def configure(self):
        self.require_service('say_hello', why='because')

    def process(self):
        self.services['say_hello']("tom")