Ability to mute outgoing and/or incoming audio

This PR follows the discussion in #198 and related [Trello card](https://trello.com/c/ne5DGmxO/201-1-as-a-user-pressing-the-mute-button-mutes-my-audio-not-the-other-person-s-audio), trying to unblock it for moving forward. 

The patch replaces the previous "mute" button by two more focused new ones: 

- the first one for muting the outgoing audio (user microphone output)
- the second one for muting the incoming audio (speaker output)

Proposed icons have been quickly handcrafted and should probably replaced by something more designed, later.

Preview:

![capture decran 2013-08-27 a 15 15 27](https://f.cloud.github.com/assets/41547/1034746/e5ffa080-0f1e-11e3-868c-615e8126b27c.png)

*(click the image for zooming in)*