Added Unicode window title support.

- Changed SFML API to receive an sf::String as the window title instead
  of a std::string (in Window::Window and Window::setTitle).
- Changed RenderWindow and WindowImpl APIs accordingly.
- Changed WindowImplWin32 to use a Unicode window title only if the
  target OS supports it.
- Changed WindowImplCocoa to always use Unicode window titles and added
  a utility function to Window/OSX/cpp_objc_conversion.mm.
- Changed WindowImplX11 to set the Unicode window title as part of the
  _NET_WM_NAME specification, which sadly is not part of the official X
  standard, but the closest anything can get. Still set regular ASCII
  title as fallback.
Simplified UTF-8 encoding procedure for X11 window title.

- Instead of first converting to a wide string and converting that to
  UTF-8, now convert to UTF-8 directly using sf::Utf32::toUtf8.
- Modify OS X sf::String to NSString helper to work for big-endian
  architectures (like OS X 10.5 on PowerPC) as well.
Fixed tiny style issues.
Merge pull request #351 from Shizmob/master

Unicode window title support.
Merge https://github.com/LaurentGomila/SFML