implemented releases per day line chart

also
- refactored growth stat display logic into common metrics
- implemented github/getreleasesbyday
- refactored date comparisons used in bucket into query helpers service

![screen shot 2014-07-22 at 2 04 27 pm](https://cloud.githubusercontent.com/assets/3188890/3662483/d5a5714c-11ca-11e4-94d7-45bcaacb55d1.png)


- trello: https://trello.com/c/Ar1VYD7A
- PR hosted at : http://dashboard.risevision.com:8000

could @ByronDarlison @tejohnso @alexey-rise review?