Removed comment on how senders are actually used

This probably will just confuse people.
Merge remote-tracking branch 'origin/master'
(Re)moved some comments on internal workings

There was a lot of stuff that most people probably wouldn't care about
in the first sections. Cleaned that up.
Added note on RAW callback

This callback is called whenever an IRC message is sent or received and
is passed that message. Useful for logging and printing to console.
Added a "Standard modules" section

This will be filled out or will direct the reader to additional
documentation later.
Initial commit of irc.lua
Fixed typo in object creation section
Made callbacks, senders and handlers replaceable

...without having to call clear_<whatever>. This was too much trouble.
The functions just return a second value if a function was replaced.
Fixed syntax error in some examples

Some examples has "string":format(...). This should've been
("string"):format(...).
Readded handler, sender, callback unsetting

This is probably a better system, actually.
Initial work on module itself; updated README

First commit of actual work on module. README now says that the module
is a work in progress.

The main module is basically complete. I just need to check for bugs,
then I can work on the standard modules.