Upgrade to manifest_version 2 for Chrome >= 27.

# Overview

As of Chrome 27, it is no longer possible to load extensions in Developer mode without `"manifest_version": 2`. And in September, it will no longer be possible for any clients to load the extension at all.

This pull request correctly sets the manifest_version and updates the `background_page` attribute to its new form. With this pull request, the application works as intended with new versions of Chrome.

### Caveats:

Manifest version 2 sets the following script policy (implicit in the manifest):

`"content_security_policy": "script-src 'self'; object-src 'self'"`

This breaks a few of the social links in the app, as well as Google Analytics.
Some users may be passing some confidential data through the app - therefore I have not changed the security policy in this pull request.

If you wish to re-enable these scripts, append the following to `application/manifest.json`:

`"content_security_policy": "script-src 'self' https://apis.google.com http://platform.linkedin.com https://ssl.google-analytics.com; object-src 'self'"`