Drop, Select, and Tether

This adds Drop, Select, and Tether, and some associated stylesheets.  

Tether is bundled with the compiled version of Drop used here. For Select, I used the coffeescript source to avoid including Tether twice. 
![screen shot 2014-03-12 at 2 32 54 pm](https://f.cloud.github.com/assets/77472/2401358/1252151e-aa15-11e3-8985-89d17168f92e.png)