Bug 1100945: adding a PasscodeHelper

This should do as an initial draft.
Looking forward to any kinds of feedback, especially lines 70-73 and 88-100. :)
In my recollection the mozSettings API is `thenable` but it's not `catchable`. So, you may need to change this if you want to catch it. This is an (IMO) API defect so it's OK to use `then(onFulfilled, onRejected)` pattern rather than `then...catch` patter here.