Adds models folder and shell files to begin RSpec testing
Tests User model associations with shoulda
Tests Course model associations with shoulda
Tests Assignment model associations with shoulda
Tests CourseMembership model associations with shoulda
Tests StudentAssignment model associations with shoulda
Adds test for has_many :student_assignments
Merge pull request #3 from O-I/rspec_models

Creates RSpec tests for the basic model associations using shoulda