Various project cleanups

- added zmtp_prelude.h (similar to czmq_prelude.h)
- added selftest methods for all classes, still empty
- added comment headers for all methods (not finished)
- added tags for man pages in public header files
- simplified error handling for memory exhaustion (asserts :-)
Simplified send/recv in zmtp_connection

- don't need separate methods for this; just call the class method
Merge pull request #8 from hintjens/master

Various project cleanups