Bucketing the donations on the your-donations page into a binned histogram

The your-donations page is great, but it's hard to get a sense of what the distribution of people's donations are.  Are most of the donations due to many small donors or a few large donors?

What I did here was I changed the histogram from showing the counts of the individual donations to showing the total values of various "buckets" of donations.

In other words, instead of showing how many $1, $2, $3, ... etc donations, it shows the total value of all the $1-10 donations, the $10-$20, and so on.

Went from this:
![screen shot 2014-07-15 at 1 00 20 am](https://cloud.githubusercontent.com/assets/1427329/3582013/1fccc944-0bf6-11e4-8260-cd8611498077.png)

to this:
![screen shot 2014-07-15 at 1 03 08 am](https://cloud.githubusercontent.com/assets/1427329/3582059/8dedb302-0bf6-11e4-82b7-11a8b9c71151.png)

With this redesign, it's clear Mayday actually has a very healthy number of small donors!  (And if I want to contribute, I should contribute at least $10)

@jueyang since you seem to have put this together initially, would love your thoughts!