Fix styling of main media embeds

We now allow the ability for embeds to be used as the article main media, but we were not apllying the correct CSS classes for these to behave correctly and responsively.

This corrects that.

##### Before
![google chrome](https://cloud.githubusercontent.com/assets/80089/3312328/577d7cca-f6d0-11e3-8187-5d09c48fcaa6.png)

##### After
![google chrome](https://cloud.githubusercontent.com/assets/80089/3312329/5dce86c8-f6d0-11e3-8af9-f767723f2b3f.png)