Added Courses, Schedules, ids, URL get support

* Added CoursesClient
* Added ScheduleClient
* Added IDs to all models. (getId(), and setId())
* get(...) call now gets data using the resource url
* getById(...) is the old get, so it uses the object's id to get data
* Updated javadocs and README.md
* Fixed a date conversion error, when the milliseconds value is 000, the field would not be displayed and the conversion would fail.