Adds option to never save master password

This option is meant for people who do not ever want to save their master password. Be it for security reasons or that they are using several master passwords. Or both.

This is realised by a checkbox labeled "Never Save Master Password" in the "General options" page that, once checked, sets the "Save password" setting to "never", deletes a possibly saved master password from memory and disk and hides the "Save password" field in the popup.

Also fixes Issue #63.