Log Message:
Update to 2.2.6:

2.2.6:

* Slight documentation changes.
  (Murray Cumming) Bug #614741 (Armin Burgmeier)

2.2.5:

* Accumulators: Allow return types that are different to the signal's.
  (Krzysztof Kosiński)
* Documentation:
  - Disable collaboration graphs in documentation
  - Use non-blurry font for graph labels.
  (Daniel Elstner)
  - sigc::hide() correction: It always only hides one signal argument.
  (Michael Hasselmann)
* Fix the MSVC++ build.
  (Armin Burgmeier)