Feature Tracking Class

Provide endpoints to Quickbooks Online Class Endpoint under the class `TrackingClass`. This is to reflect [Quickbooks description](http://quickbooksonline.intuit.com/screen_class.jsp) of classes. 

Alternative names could have been `Clazz`. I opposed `Clazz` as its not a common replacement for `Class`. `Klass` would have been a better substitution.