Add 'answered_requests_with' method to Ask
Change average and median response time methods to use MentorAsk#answered_requests_with method
Change average and median response time functions to use MentorAsk#answered_requests_with method
Move response time methods from Location and Category to Statistics concern
Split Statistics tests into own file and reference the shared specs in location specs
Fix issue of mean and median failing to properly handle empty data sets
Change specs for Category to use shared statistics specs