Switched to new zsys logging facility

- removed all use of zlog class
- see CZMQ issue #519
- note ALL engines need to be regenerated
- removal (not deprecation) of zlog forces the change

zsys_debug/error/etc. ensure all log data is sent to same place.
This means no longer using printf/zclock_log etc. in application
code.
Merge pull request #67 from hintjens/master

Switched to new zsys logging facility