Fix eviction of saved password when saved encrypted on disk

Fixes issue #63:  https://github.com/bitboxer/chrome-passwordmaker/issues/63
Implement option to disable master password saving

This is realised by a checkbox 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
Remove forgotten debug statement
Merge pull request #83 from oolongbrothers/disable-password-saving-option

Adds option to never save master password