Update css

Currently, the HTML/CSS isn't super flexible, especially for things like long consent forms, for example:

![image](https://cloud.githubusercontent.com/assets/83444/5345487/e692cc42-7ecc-11e4-8e8f-3fbd6c0a8792.png)

Similarly, for logos that aren't the same size as the placeholder, it doesn't look very nice:

![image](https://cloud.githubusercontent.com/assets/83444/5345718/b28dd016-7ece-11e4-8813-d3cce9fe2906.png)

This PR tries to make the HTML and CSS a little more flexible so they won't need as much tweaking if your experiment doesn't fit within the same dimensions as the example. Mostly it takes advantage of the [media bootstrap component](http://getbootstrap.com/components/#media) (for dealing with the university logos) and then removes the gray border from the various containers and doesn't make their position absolute.  Here are some screenshots of what it looks like.

The ad:
![image](https://cloud.githubusercontent.com/assets/83444/5345505/031ff8b2-7ecd-11e4-8fe8-8e3cf73b91b9.png)
![image](https://cloud.githubusercontent.com/assets/83444/5345730/cf24a13c-7ece-11e4-8a21-f05dfe91222f.png)


Consent:
![image](https://cloud.githubusercontent.com/assets/83444/5345508/0d96831a-7ecd-11e4-8584-e52617733613.png)

Instructions:
![image](https://cloud.githubusercontent.com/assets/83444/5345510/13f8fbf2-7ecd-11e4-91c5-afd3e64a913b.png)

Experiment:
![image](https://cloud.githubusercontent.com/assets/83444/5345513/192a28da-7ecd-11e4-9b0a-8a861eee22b9.png)

Post questionnaire:
![image](https://cloud.githubusercontent.com/assets/83444/5345517/1e989f54-7ecd-11e4-9104-08beda4ca01b.png)

Finished:
![image](https://cloud.githubusercontent.com/assets/83444/5345519/22b161b6-7ecd-11e4-8135-438c6556e7b4.png)