Add Airbnb to #used_by companies

Would you consider adding Airbnb to the home page, or is this topic too sensitive?

If this is up for discussion, any feedback on this commit? Would anyone want to help out?

I thought about arranging the company images to an in-line style instead of in two columns, but I didn't want to get too much into editing if the topic is off limits.

Here's a screenshot:  
![screen shot 2014-11-27 at 2 12 44 pm](https://cloud.githubusercontent.com/assets/4951910/5222050/9b6aed84-763f-11e4-91e9-2af12e223d1a.png)