added: util/{Signal,SignalAction}: light-weight generic signal impl.

Don't feel like writing a whole lot about them now, but here are the
basics:

  * Signal implements a generic version of the callback mechanism used in
    Scheduler, with the option to use blocking callback invocation
    instead of using Boost.Asio for asynchronous invocation.

  * Signal use should (*should*) be thread-safe in terms of accessing a
    signal object from different threads.
ScheduledAction: fix friend declaration for std::allocator implementations

This fixes a compile error with Clang, introduced in @370ec08.