cppbind: rework to allow binding nearly everything.

cppbind: rework to allow binding nearly everything.

It is now possible to register all types of member function
and free functions as methods for nasal::Ghost objects.
The return value and arguments are converte automatically
to the required types.

Also usage is simplified by removing replacing the old
method and method_func with a single method function
which only needs a name for the method and something
callable.