Update Signals.h

- Added to Connection<>: operator()(...), block(), unblock() and blocked() methods

- Added to Signal<>: block(), unblock() and blocked() methods

- Changed in Signal<>: operator(...) now calls Connection::operator()(...) instead of Connection::delegate()::operator()(...). This was necessary to make connections blockable

- removed Arduino header again because these classes should work with any compiler that supports C++11.