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.
So the `has_one` function defines an association between different models. In this case, it is a polymorphic association meaning that it can be used by multiple models. Both students and startups should be able to sign-in and have sessions, but students and startups are inherently different models.

Spend some time and read about database associations, and in particular, [polymorphic associations](http://guides.rubyonrails.org/association_basics.html#polymorphic-associations).