Create TrackingClass model and service

Class is saved. Decided to use a more verbose description of "class".
Describe TrackingClass.to_xml_ns
Merge pull request #66 from nickhammond/addressable-cleanup

remove redundant methods already mixed in with addressable
Merge pull request #67 from whereisciao/feature-tracking-class

Feature Tracking Class
Merge pull request #67 from whereisciao/feature-tracking-class

Feature Tracking Class
Merge commit 'ef1d32c6b17bbd75b873051bfb7329ab56238d67'
removed commented out and unused requires
Updated README to clarify how to do filtering on datetimes
Updated README to clarify how to do filtering on datetimes
Updated README to clarify how to do filtering on datetimes
Update README.md
* Desktop: Begin implementation of enforcing filter ordering. The Intuit
* API requires that filter elements obey a given order. We don't expect
* callers to know this order. Thus, callers can give us an array of
* filters and the Service class will re-order them appropriately. As of
* this release only `Job` service has `FILTER_ORDER` defined. However
* going forward its easy to add it to other Service objects as needed.
Updated README to break out supported Online / Desktop services into their own tables