SWIG wrappers for siglot only supported unary signals until now. This
commit adds support for all remaining signal arities to make it possible
to connect to arbitrary signals from Lua.

The implementation makes heavy use of templates but, due to the
limitations of VS 2010 (no variadic templates), still contains lots of
duplicated boilerplate code.

`sigslot.i` has been moved from `scripting/` to `ext/sigslot/` to keep
it close to the code it's wrapping.

References #1