Signal.callback

I used some c++11 magic and renewed the signal and callback classes. They are now classes with variadic templates and accept as many arguments as needed.
Also removed the Slot(-container) class, because it's useless in my opinion.
Maybe some documentation could be made.

I know that this is a big cut, because I removed the Slot class, but really, you don't need it. I understand if you don't accept the new signal class, but at least the callback thing - commit 6e9a8e2, a8cdc7f, f5d02b3, d060716, 9ec1533 - aren't breaking any API (at least it shouldn't).
Let me hear your doubt about it.