add plagiarism api hooks to other areas of Moodle code

The Plagiarism_API currently only hooks into the Moodle Assignment upload types - as part of a GSOC project a student will extend this to work with other Moodle modules (linked as subtasks)

http://docs.moodle.org/dev/How_to_add_support_for_a_Plagiarism_Plugin_to_my_activity_module