New design for visualization

While working on other visualizations, I revisited to this one. Main changes:

* Replaced our "green" theme of Bootstrap with the default Bootstrap theme (closes #76)
* Updated the structure of the HTML
* Better mobile display

Before updating the [live visualization](http://enram.github.io/bird-migration-flow-visualization/viz/), I'd like your feedback.

This is how it looks now: 
![image](https://cloud.githubusercontent.com/assets/600993/4247342/87d345b4-3a58-11e4-9c27-50727d6259ff.png)

And on mobile:

![image](https://cloud.githubusercontent.com/assets/600993/4247724/028af4e6-3a5e-11e4-92e1-81e84f92cd91.png)