Added Courses, Schedules, ids, URL get support

* Added CoursesClient
* Added ScheduleClient: get and getList not working yet, as API doesn't have the 'scheduleentries/' endpoint defined. Will fix that tomorrow.
* 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