Bug 1008934: a more responsive UI for mobile & desktop

Related bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1008934

*Hint: “Démarrer l'appel” is French for “Start call”.*

Start page, portrait on mobile:

![screen shot 2014-05-13 at 16 26 35](https://cloud.githubusercontent.com/assets/41547/2958634/622b409e-daab-11e3-9a77-1dba7ef4480b.png)

Start page, landscape on mobile:

![screen shot 2014-05-13 at 16 26 41](https://cloud.githubusercontent.com/assets/41547/2958640/7028777a-daab-11e3-82f0-1b4312347d43.png)

Conversation, portrait on mobile:

![screen shot 2014-05-13 at 16 25 06](https://cloud.githubusercontent.com/assets/41547/2958657/82ae12ba-daab-11e3-9414-0e2e39325146.png)

Conversation, landscape on mobile:

![screen shot 2014-05-13 at 16 24 58](https://cloud.githubusercontent.com/assets/41547/2958659/8b68e7a4-daab-11e3-92a2-e047c66e13b8.png)

Start page, desktop:

![screen shot 2014-05-13 at 16 26 21](https://cloud.githubusercontent.com/assets/41547/2958663/9a04c940-daab-11e3-829e-759a7a1d3c2f.png)

Conversation, desktop:

![screen shot 2014-05-13 at 16 25 44](https://cloud.githubusercontent.com/assets/41547/2958664/a14d43b2-daab-11e3-93b0-8b664e7f74e5.png)