Split CTCP into CTCP and CTCP_REPLY, cleanup
Update PING to return sender to callback
Update README, add CTCP information

In addition to adding information about the new way CTCP works, the
"Handlers" headers for each standard module were renamed "Callbacks", as
this is what the sections are really describing.
Remove test file

It did not provide much use for users of the module. I will keep it
locally now.

An example file may be added later, but the README is probably
sufficient.