Link in libuuid
Fixed #322
Additional files for issue #322
Fixed #325

- zcert_new returns NULL when libsodium isn't installed
zproxy did not build with pre-4.x versions of ZeroMQ

- for versions prior to 3.2.1, use zmq_device instead of zmq_proxy
- does not implement capture socket on these older versions
zproxy fix for older versions was not working

- selftest did not work properly with zmq_device as underlying engine
- conditional compile was using wrong macros
Merge pull request #324 from hintjens/master

Fixed #322