Added plugins directory and corresponding Makefile

The plugins directory contains a makefile that calls a bash script
that compiles the plugins (not neat, I know) and subsequently
moves the shared objects to ../bin/plugins.

It also contains common.c, a plugin that is required by the bot
that contains (or will contain, anyway) a couple of essential
commands ('join, 'part) and a couple of message handlers
(NOTICE, etc)