Make options page refresh depend on page state

Make minimalOptionsRefresh depend on the current state of page, not on
stored variables. Also, remove unused pageOpen parameter.
Sync support

New option to sync settings across browser installations if Chrome Sync
is enabled.
Allow sync data to be wiped

Include a reset button for the Sync option to clear stored variables
from Chrome Sync and disable this extension's Sync feature across all
Chrome installations.
Add underscore.js 1.6.0

Copyright only license.

Removed debugging line: //# sourceMappingURL=underscore-min.map
Debounce saveOptions for text input fields

If doiResolverInput and shortDoiResolverInput are typed in manually
(i.e. not copy/paste), they can fire saveOptions rapidly. Debounce them
so that Chrome Sync doesn't too many sync requests.
Version 2.2.0