Adds Markdown support to Transactional Email copy for Campaign Group

Sample input:
````
Hi I am just a robot

- hello world
- right back at ya

OK me neither [Hi there](http://www.dosomething.org/campaigns)
````

Resulting email:
![screen shot 2014-11-25 at 4 32 56 pm](https://cloud.githubusercontent.com/assets/1236811/5191967/07280186-74c1-11e4-85e0-27d8fae3d7ff.png)