Cleanups to zyre_event class

- use enum for event type
- fixed method names to be a little more readable
- fixed various memory leaks
- wasn't getting group name for JOIN/LEAVE
- fixed comments in header file, used for man page
- added minimal explanation of class in header, for man page
- added man page to doc/Makefile.am and generated zyre_event.txt
Merge pull request #125 from hintjens/master

Cleanups to zyre_event class