Updated INITIATE command for extended vouch design
Made work over ZeroMQ v4.0
Removed keypair and keystore classes

* These are now supplied by CZMQ's zcert and zcertstore classes
* Fixed curve_server test case, which was not starting any clients
* There was an error in curve_server, sending messages to ->control not ->data
Merge pull request #14 from hintjens/master

Implemented modified INITIATE command