I understand that ZeroMQ 3.2 has removed the HWM socket option. In fact, when running my own code that uses em-zeromq under 3.2 I encountered crashes that I could track down to this line.

However, according to the [3.2 upgrade guide](http://www.zeromq.org/docs:3-1-upgrade), the new recommended way of setting the `ZMQ::HWM` option is to set both `ZMQ::SNDHWM` and `ZMQ::RCVHWM` at the same time. In the name of backward-compatibility, I would prefer it if em-zeromq retained the `Socket#hwm=` method, which would either carry on setting `ZMQ::HWM` under 2.x, or set both of the new constants under 3.2. On the other hand the attribute reader `Socket#hwm` can presumably be removed from 3.2 as it is effectively meaningless.

I would also like to see `ZMQ::SNDHWM` and `ZMQ::RCVHWM` mapped like all the other constants in this file (perhaps with slightly less hideous names), although it's something that can be done later in the process of adding full 3.2 support to the gem.