2.99.4 (unstable):

* Minor documentation improvements.
  (Murray Cumming)
* Make some constructors explicit.
  (Murray Cumming)
* Some more minor uses of C++11 syntax.
  (Murray Cumming)
* Remove unused SIGC_FUNCTORS_DEDUCE_RESULT_TYPE_WITH_DECLTYPE.
  (Murray Cumming)
* Reformat code with clang-format.
  (Murray Cumming)