added ENV varible to explain what partials are being used

I thought I'd pass this along in case you guys wanted to use it in Sufia. It gives you html comments that explain what view partials rendered the various parts of your page. You can see it in the dom inspector below. You just need to run your server with an environment variable set: ```EXPLAIN_PARTIALS=true rails s```
![image](https://cloud.githubusercontent.com/assets/74879/2951881/835aeea8-da2f-11e3-9488-a2dd1f53526b.png)