lua: Added logging utility functions.

There are four new functions callable from monitor source and lua destination.
They are syslogng.debug, syslogng.error, syslogng.verbose, syslogng.info.
Through them it is possible to create syslog-ng log messages. All of them
requires one string (the message) as a parameter.

Signed-off-by: Tusa Viktor <tusavik@gmail.com>
Merge pull request #22 from talien/feature/lua-utility-funcs

lua: Added logging utility functions.