Add milestone widget in insights

For issue https://github.com/uberVU/elasticboard/issues/84

### For a milestone that is not behind schedule
![screenshot from 2014-03-02 11 38 19](https://f.cloud.github.com/assets/810040/2303885/a3914fd8-a1ee-11e3-8340-01781fffba5a.png)

### For a milestone behind schedule
*Blue means not due date set.*
*White one means no issues completed 0% progress*
![screenshot from 2014-03-02 11 38 04](https://f.cloud.github.com/assets/810040/2303886/a391d49e-a1ee-11e3-9d36-1fcacc00e62a.png)

The API also provides data about the labels associated with the release. I don't see any valuable info by adding those unless they actually link to the issues. Thoughts ?