using gem to export excel
erase unused file
check invalid format is in and show it
Merge branch 'develop' of github.com:DonutWorks/Ari into refact/excel-export-fit-to-all-format

Conflicts:
	Gemfile.lock
	app/controllers/admin/export_excel_controller.rb
	app/views/admin/export_excel/create.xls.erb
	lib/html_comment_parser.rb
Merge pull request #178 from DonutWorks/refact/excel-export-fit-to-all-format

#138 using gem to export excel