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
Fix prev_month syntax
Make everything better

* Dates reflect incidents on map even if they cover less than query
Remove unnecessary self, private
Improve use of JS
Improve handling of dates
Merge pull request #12 from HayleyCAnderson/heatmap-expansion

Allow inclusion of 6 months of data in heatmap

* Fetch and use past 6 months
* Improve accuracy of legend dates