Adding Active Cover and Cover Actions

@veskuh
Still work in progress, but your reviews are welcome.

I have added following cover actions :

- Sliding left refreshes timeline, mentions and direct messages
- Sliding right opens new tweet window

Also the cover shows total unread count for timeline, mentions and direct messages in top right corner.

TODO:

- Add main content in active cover
- Show a different active cover and actions when user is not signed in