Donation histogram: show count of donations AND value of donations

Making donation histogram able to switch 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)