Fixed sender table description in README
Combined callback and handler call in one function

This allows calling of other handlers/callbacks from a handler, eg. call
CTCP handler/callback from PRIVMSG handler.
Gave utility functions a distinct header
Added basic CTCP handler and associated test
Removed extra newlines in irc-engine.lua

There were some for some reason.
Put note about handler returning nothing

If the handler doesn't return anything, the corresponding callback won't
be called. This is to allow passing on the processing of a message to
other handlers without triggering the callback of the original handler.
Added CTCP and ACTION handlers
Added tests for CTCP and ACTIONs