problem: there is no ability to set or get socket options.

In order to determine if using gsl for generating socket options is beneficial, we need to determine how we would like to set, get, and test options.

* refactored czmq.go into zsock.go and zsock_options.go for better code organization.  Having the socket options in a separate file is beneficial if we decide to use gsl to generate the socket option code.
* added a couple of options and tests as a basis for discussion about gsl and whether it would be beneficial to use it.