ZeroMQ support

[jbrisbin]

Following the mention of Kafka support in #6, it would be useful to many people to support ZeroMQ as a transport as well. It has well-established non-blocking usage patterns that would map very easily onto Reactive Streams.

Having more than one transport supported OOTB would also serve the very useful purpose of guiding abstractions that need to live in higher-level modules like the tcp and core modules.

[NiteshKant]

@jbrisbin are you referring to zeroMQ codec support similar to what spotify provides here over netty or a ZeroMQ client & server SPI?

[jbrisbin]

ZeroMQ could function as a transport library just like Netty, the only meaningful difference being the real data type passed into the connection Subscriber. In the Netty case, that would be a ByteBuf and in the ZMQ case, that would be a Frame.

[jbrisbin]

This also illustrates the need to handle a Buffer of bytes in two radically different ways but from a common abstraction. :)