Added a missing pseudo-argument
Added TODO about overwriting handlers/senders
Handlers/senders now get the object as an argument
Simplified explanation of sender functions
Added information on handler/sender overwriting

I went with "returning false/nil and an error code" since these errors
aren't easily avoidable, especially when modules are introduced.
Added TODO about function access to IRC objects

Currently handlers will be allowed to access the full module, but this
could be a security hole. The alternative is to fully sandbox them and
just expose certain parts of the object to them (and probably only
certain parts of the Lua standard library). I will think about this.