Fix datetime labels on histogram

Factor out histogram calcs into HistogramArray.
Remove report formatting from helper and put in new
PORO facade class, ReportFormatter.
	modified:   app/controllers/surveyor_gui/reports_controller.rb
	new file:   app/facades/report_formatter.rb
	modified:   app/helpers/surveyor_gui/report_helper.rb
	modified:   app/views/surveyor_gui/reports/_report_data.html.haml
	modified:   app/views/surveyor_gui/reports/_show_report.html.haml
	modified:   lib/surveyor_gui/engine.rb