Signin & signout

"rails generate controller Sessions new create destroy"
"rails generate migration AddRememberTokenToTeachers
remember_token:string"
"rails generate migration AddRememberTokenToStudents
remember_token:string"
"rails generate migration AddRememberTokenToAdmins
remember_token:string"
"rake db:migrate"
validators applied to teacher, student, and admin to ensure email
uniqueness across multiple tables