New webservice: add/update assignment in course

We need an add/update assignment to course webservice.

It should hold all the parameters/options you can set for an assignment: due dates, grading scales, etc.

We would even like to have the option to add student and teacher roles to this assignment.

This way we can use the administrative system as a source for assignments in Moodle. In the administrative system it is already known which assignments a student needs to make and which teachers should grade the assignments.