Version and readme updated - some minor code cleanup.

Found out about about:memory?verbose and minimize memory usage
to debug memory usage.  This helps in combination with
about:compartments.
Fixed unloading issues and added dynamic redraws

The preference observer wasn't unloading.  Referencing it as an element
of the window object rather than on the global scope fixed the problem.

When preferences are changed the currently open pages will have their
dimmer overlays redrawn to the new opacity.

When exemption is toggled for a domain, the currently open page will
have its overlay updated.  This should actually updated all currently
open windows and tabs because pages from the same domain could be open
in different tabs.
Toggling domain exemption updates all browser tabs in current window.
Fixed pref file error on new install and added overlays on startup.
Unloading addon will now reset existing tab browsers.
Updated version
Updated version