Renamed some functions for consistency with Qt signals.

Since the ControlSignal is inspired by the Qt signal, we're going
to share the same language.

* addSlot    -> connect
* removeSlot -> disconnect
* slotCount  -> receivers
Prohibit variadic signals/slots; change arity to paramCount

The use of variadic signals was a bit confusing and sort of defeated
the protection provided by the arity checks. I don't have any immediate
need for them, so I'm removing the functionality. Variadic slots have
also been removed. I could provide a flag to enable disabling arity
checks on a per connection basis, but I don't think that variadic
slots are going to be of much use without variadic signals.

The commit also renames arity to paramCount. It now represents the
number of parameters *excluding* the callback, which is implied
and obligatory in all situations.
Minor documenation revisions
Update HTML docs

The HTML has gotten out of sync with the comments from which they
are built.