added break-word wrapping. fixes #2728

There are two ways to make this better - 

1) https://github.com/gratipay/gratipay.com/issues/2728#issue-41956593

2) Let the wrapping break long words

This is an implementation of the second. (I think that the second option breaks less of the layout.)

![screenshot from 2014-09-06 22 36 56](https://cloud.githubusercontent.com/assets/3893573/4175848/c3d47c3c-35e8-11e4-8bd7-e955fbc8b22b.png)