PROGRESS: simple duration tracking

- content script sends regular messages, with no end-condition except
  the closure of the tab, and the background page updates the
appropriate page visit in NewPageVisits
- problems
  - no end condition
  - doesn't check if tab is active
  - if it doesn't print out the page visit it finds on reception of the
    content script's message, things somehow don't work. Backbone
localStorage seems to be time sensitive somehow
  - console logging