Added module Context.get/set methods for get/set of default context options

As dicussed in #204, setting the context options programatically is much desirable (rather than via environment variables). I added the following functions to `exports.Context` as such:
* zmq.Context.setMaxThreads(value)
* zmq.Context.getMaxThreads()
* zmq.Context.setMaxSockets(value)
* zmq.Context.getMaxSockets()

Feel free to suggest changes to the API. Also, as I am fairly new to creating addons, feel free to set me straight.

(P.S. travis build failure might be because of the known test issue during unbind #315)