New installer .nsi with Uninstall warning message.

To warn user that they must save their .con files, if they were
stored in the CONFIGUATIONS subfolder.

Message reads:

  !define MUI_UNCONFIRMPAGE_TEXT_TOP "!! W A R N I N G !!    \
  >> Read this first. <<   \
  *Uninstall* will remove ALL FILES from the folder below, \
  INCLUDING any of your own .con files \
  created in CONFIGUATIONS or other sub-folders. \
  YOU MUST SAVE your own files BEFORE \
  you press the *Uninstall* button."
Merge pull request #9 from wjcroft/master

New installer .nsi with Uninstall warning message.