MDL-30085 core_grade: added a grade web service
MDL-30085 Fixing coding style
MDL-30085 Missing and invalid comments fixed
MDL-30085 Rename core_grade component references to core_grades

Also, change since tags from 2.6 to 2.7 and fixed invalid comments
MDL-30085 Added missing MOODLE_INTERNAL and MUST_EXISTS check