Set up initial PluginManager

PluginManager sets up a single loadable package 'irc' and loads all .lua
files in the plugin directory.
Implement IRC Lua event dispatch
Fix wrong index in lua_setup_packages
Add an error handler for plugins

Use debug.traceback() to get a stack trace for errors.
Support building with DEBUG=1
Fix bug in lua_addhandler()
Expose the Conn to Lua for events