Support Pub/Sub

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Refs #2.

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Reliably matching when an incoming message is actually a pubsub event and when it's a response to a query (MGET mostly) turns out to be harder to than expected.

Some other implementations take the simple route and just assume every response that looks like a pubsub message must be one, like then-redis.

See 2f0ba2e for a test case that would fail in this case.

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Reliably matching when an incoming message is actually a pubsub event and when it's a response to a query (MGET mostly) turns out to be harder to than expected.

Turns out things aren't really that complicated either.

The SUBSCRIBE and UNSUBSCRIBE commands support a variable number of arguments and return a non-standard answer when not using a single argument. However, their operation is quite simple when only considering single arguments. As a first step, we should focus on supporting this simple use case.

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The error'ed travis build is being addressed by #25.