Restored <em:unpack> to install manifest

The Mozilla Add-ons validator says reading the extension’s contents out of a .jar is slow when the extension remains packed up in an .xpi. A .jar was required before Firefox 4, so we need to keep the overall extension unpacked until we drop support for Firefox 2.x and 3.x.

This reverts much of commit ea55e57133d1b8917cac79e1e1dc754f87e2eee3.
var → let
Updated Mudim monitor

Mudim monitor posts to the browser’s notification bar in versions of Firefox that have about:addons.

Fixes #32.