Video fallback

- [new] Videos in video pages work in browser that don't support HTML5 video (flash fallback)
- [new] Fluid media container (thanks @indeox for the very useful mixin)
- [new] Feature tests

PS: for some reason the flash fallback doesn't seem to load the poster. Will investigate further later.

![screen shot 2013-10-18 at 16 36 54](https://f.cloud.github.com/assets/85783/1361665/4599abe8-380b-11e3-8ebd-f7ab72ee1b5b.PNG)

![ ](http://media.tumblr.com/tumblr_mekc6kZTap1raprkq.gif)