Allow inclusion of 6 months of data in heatmap

* Heatmap will be more accurate/useful with more data
* Fetch and use past 6 months of data for heatmap
* Reflect time period and count in heatmap legend
* Allow API query to start in June
What about something like `6.months.ago.strftime("%FT%T")`?

The `6.months.ago` call will make that number dynamic so it'll always reflect 6 months ago from today. And the `strftime`call will just format it in the way that SODA requires it. 

http://apidock.com/ruby/DateTime/strftime