add ducksboard metrics routes

Adds three routes for ducksboard:
- /mau
- /monthly_checks
- /leaderboard

To make these:
![screen shot 2014-12-26 at 3 30 15 pm](https://cloud.githubusercontent.com/assets/2533112/5559956/f57656be-8d1a-11e4-8834-0602d591a429.png)

I pulled some common things into a :before_filter but otherwise this doesn't address performance or fix the index page or anything.