Create Worker Client class

[wglass]

Somewhat analogous to the 'Processor' client in KIP-28:

https://cwiki.apache.org/confluence/display/KAFKA/KIP-28+-+Add+a+processor+client

Should be a grouped (with the 0.9 api) client that takes a set of input topics and one output topic, plus
a function used to process the messages.

from kiel import clients

w = clients.Worker(
    input_topics=("events.foo", "events.bar"),
    output_topic="events.bazz",
    processor=aggregate_counts,
)

Things to consider:

What sort of return value should the processor have?

It should probably actually be a generator or gen.coroutine. It's liable to be called several times before
having an output, especially in the case of aggregators.