Bitcoin-Qt: add a Reset button to the options dialog

- a click on "Reset Options" sets all options to the default values by
  removing all stored settings (QSettings), loading the defaults and
  saving them as the new settings
- before the reset is executed the user is presented a confirmation dialog
- special casing was needed for StartAtStartup
Merge pull request #1685 from Diapolo/Qt_add_options_reset

Bitcoin-Qt: add a Reset button to the options dialog