Donation histogram: show count of donations AND value of donations

Making donation histogram animate between number of donations in each bin and value of donations in the bin.

![screen shot 2014-07-19 at 10 09 16 am](https://cloud.githubusercontent.com/assets/1427329/3635278/8043b498-0f68-11e4-8983-d7fca2a9a640.png)

Replaces pull request #104 -- rebased that pr to incorporate people's latest css tweaks on the histogram page