ProfessorCourse DAO implemented.
Working DAO for CartTuple
Fix db connection bug--was not closing database connection after
initializing each table.  Fixed.
Changed ProfessorCourseDAO api.  Removed model object.
Updated APIs of CartDAO.  Removed model object.
Added remove() to CartDAO.
Merge pull request #17 from nathanshih/ProfessorCourse

ProfessorCourse DAO implemented.
Fixed merge conflicts
Merge pull request #18 from nathanshih/cartTuple

Working DAO for CartTuple
Fixed merge conflicts