Bitcoin-Qt: better copyright year handling in AboutDialog

- this change allows us to keep the translation without the need to
  re-translate any string, when we update the copyright year
- copyright symbol is changed to HTML to ensure we get no encoding
  issues and it's removed from the translation string so translators don't
  break it by mistake
Merge pull request #2258 from Diapolo/Qt_copyright_about

Bitcoin-Qt: better copyright year handling in AboutDialog