Update message passing

* sendRequest is deprecated, change to sendMessage
* Group all message handling in background.js
* Condense enable_context and disable_context into one request which is
  accompanied by an enable/disable setting
minimalOptionsRefresh depends on page state

Make minimalOptionsRefresh depend on the current state of page, not on
storage.
Sync support

New option to sync settings across browser installations if Chrome Sync
is enabled.