Update to 2.2.13

Changelog:
Version 2.2.13 (9 May 2016)
--------------

        Use glib's unix fd implementation for io_watch where glib >= 2.36
        is installed, unless otherwise chosen during configuration
        (acinclude.m4, configure-gtk2.ac, configure-gtk-3.ac;
        io_watch.cpp).

        Update documentation (callback.h, extension.h, future.h).

Version 2.2.12 (18 October 2015)
--------------

        Simplify Cgu::Application::add() method (application.cpp).

        Minor implementation improvement for
        Cgu::Thread::parallel_transform* functions (do pointer arithmetic
        before not after tasks are launched) (parallel.h).

        Permit return value optimisation when returning thread object on
        starting new thread (thread.cpp).

        Update libtool to version 2.4.6.

        Documentation fix for doxygen-1.8.9 (docs/Doxyfile.in).

        Update code comments (emitter.h).

        Update documentation (doxygen.main.in, application.h, emitter.h,
        extension.h, future.h, future.tpp, shared_handle.h, shared_ptr.h,
        task_manager.h, thread.h).

(ryoon)