Cleaned up README a bit
Allow CTCP params to a table or a string
Fixed NOTICE CTCP handling; removed comment

The comment was about not returning values from a handler to not call
the associated callback. That is covered in the main file, and will be
added to the README if it isn't there already.
Finished documenting standard modules
Added note about callback calling to README
Made prefixless messages' sender an empty table

This should make it easier to check if there is no sender while not
breaking things that try to access the sender table.