Add student role

Student model added. It has a polymorphic association with User thus can sign on, have sessions, etc. It has some extended attributes as can be seen in the schema and specs.
Index speeds up querying of database. Since we will be querying for email often, this is a a good column to index.

[More info on indexes here](http://dev.mysql.com/doc/refman/5.0/en/mysql-indexes.html)