login.php enhanced with teacher login

is_teacher_valid() added to teacherModel.php
Session variable "role" introduced

"visitor" granted reading access to contactus and syllabus pages,
"student" granted reading access to lectures and assignments pages,
"teacher" granted writing access to assignments page and reading/writing
access to students page, as well as role-based customization of header
and navigation bar
Admin role introduced

along with courses.php allowing admin to assign teachers to courses