add generic signal class
removed unneeded files

instead of the old slot class you should now use a callback (instance,
reference, pointer, whatever). You can do exactly the same things as
with the Slot class. You can enable (connect) disable (disconnect) and
destroy (clear) it.
add operator== to callback.h
fixed using typo in signal
callback.h: fix template deduction

the compiler (clang 3.4) cannot deduct the template argument for the
constructor. So instead of delegating the c'tor implement it. I don't
know if one can fix the original problem, but this will solve it for
now.
signal.h: fixed invoke method

I forgot to add the const specifier.
add signal.h to makefile.am
changed all old signal handler to the new one

In fact it just was removing some slot/slot variables and use the new
initializer lists (they are awesome {!})