Map + Toggable graph

In this PR:

1. We've added a map showing donation counts and amounts aggregated at zip code level. We've included an overlay of congressional district to show the correlation.
![](http://f.cl.ly/items/1X3m3D000q1f2B3w3z25/Screen%20Shot%202014-07-02%20at%2012.54.43%20AM.png)

2. The graph now toggles between amount of donation (in ascending order) and counts of donation (in descending order), showing that small donations dominate.
![](http://f.cl.ly/items/2n2i0x2a3T0y3Q0C2a27/animation.gif)

3. Data with no amounts (`NaN`) is being filtered out.

cc @millerEric @ianschuler