added set and get functions on ZMQ context

Hi,

I felt myself wanting to modify the number of sockets in a context, so I added this quick patch for lua-zmq. It would break compatibility for older versions of ZeroMQ (using zmq_ctx_set from 3.x), so no worries if you don't want to accept it for that reason. 

I'm not familiar enough with the binding generator to drop in conditional compilation (or conditional code generation?), but if there is a way to decide what code to build based on the version of ZeroMQ you're building against, that would be ideal.

Tim