Use of html5 cache mechanism to provide offline mode

I have added a cache manifest file. It is generated with jinja to put the version number. I have also modified the check_version() function to update the cache instead of checking version.json. Finaly, I have added a listener to the "updateready" event to inform the user.

Now the application can work in offline mode. If an update is ready, the cache manifest is updated consequently and the application warns the user.

The only negative point is that the twitter and facebook buttons doesn't appear in offline mode (but they don't have many sense in offline).