make a hash of default dates for reports to use
default end date to today
daily sales by category report
updated description
doesn't plot days where there are no sales
add $ to y-axis labels
reorder highcharts api options to match documentation (alphabetical)
Merge branch 'feature/daily_sales_by_category_report' into develop
Merge branch 'feature/reports_tweaks' into develop

Conflicts:
	app/controllers/admin/reports_controller.rb
Merge branch 'develop'