Add a failing test indicating errno bug.

Errors returned from C functions are not always equal to the constants
defined in zmq.go.
Correct errno bug in casterr.

Errors are now rightly comparable to the constants.
Merge pull request #68 from jtacoma/errno-comparison

Fix Errno Comparison.
add a test for capture=nil in gozmq.Proxy
fix gozmq.Proxy with optional capture socket = nil
Merge pull request #69 from PhiCode/master

fix gozmq.Proxy
Delay zmq_init until it needs to be called.

Don't call zmq_init right away; wait until NewSocket requires it.
Add Context.Set() and Context.Get() for options.
Test Context options and fix for zmq_3_x.
Catch init errors for zmq_3_x context options.
Document Context.Get() and Context.Set().
Merge context options.