2.99.13: (unstable)

* Replace the deprecated std::result_of with std::invoke_result
  (Kjell Ahlstedt) Issue #19

Build:
* CMake: Supply SIGC_BUILD and use CMAKE_CXX_STANDARD
  (Jeff Trull) Issue #23, pull request #24

Documentation:
* Remove some obsolete documentation
  (Murray Cumming) Issue #12 (Jeremy Bicha)
* Rename README to README.md and update it
  (Murray Cumming) Issue #18 (Traumschule)
  (Jürg Billeter) Pull request #20
* Reference docs generated by Doxygen 1.8.11.
  Hopefully it will show up at
  https://developer.gnome.org/libsigc++/unstable/

Tests:
* Add some necessary #include directives
  (Lennard Berger) Pull Request #15