zproxy api adjustments + test improvements

I adjusted the zproxy api to be more similar to the zsocket api - it now uses a zproxy_bind call that accepts the three endpoints (front, back, and capture) similar to zsocket.  When passed a zproxy created with zproxy_new, this call spawns the attached thread and signals the proxy to start. 

I feel that this will mesh better with the way people work with zsocket now (creating a socket, setting it's options, and then binding it) once I start adding more options to the proxy.