Update Media doc with move and copy functionality

Update the [Media doc](http://wistia.com/doc/media) with the new, awesome Move & Copy functionality!

yay @jamesfzhang :fireworks: :fire: :fire: :fire_engine: 

![screenshot 2014-03-25 11 17 56](https://f.cloud.github.com/assets/651753/2513541/c77529d4-b430-11e3-9780-7b15cbbd4d13.png)