Adding Active Cover and Cover Actions

[siteshwar]

@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

[siteshwar]

Completed following items:

• Show last 2 tweets from timeline in cover
• Show different cover when user is not signed in

I believe this can be merged now.

[veskuh]

Excellent. Thanks.