Ability to specify protocols for a specific step via MRStep

[tarnfeld]

The ability to more explicitly assign an input/output/internal protocol for a step when in the Job.steps() method would be great. For example;

def steps(self):
    return [MRStep(mapper=None, reducer=None, output_protocol=FooBar),
              MRStep(mapper=None, reducer=None, output_protocol=Baz)]

The input_protocol to the second step would be inferred by the fact the output_protocol has been set on the previous step.

[coyotemarin]

This especially makes sense if some of your steps are JarSteps.

[coyotemarin]

Updated to use MRStep rather than self.mr(), for consistency with #815.

[coyotemarin]

Also pretty topical now that we're working on Spark.