Fix relations of Tutor/Client and User
Create client and tutor models.

First step in implementing appointments.  The Tutor and Client models
allow for a given User to be both a Tutor and a Client to some other
Tutor.

Ex:  I may tutor math, and be tutored in chemistry.

The other implementation option is an appointment model with multiple
foreign keys to User.  However, we have additional metadata to store
with tutors (i.e., subjects tutored), which supports using an extra
table.

(Also, multiple FKs to the same table are ugly in Rails.)
Bare Appointment model.

Just including the relations with Tutor and Client for now.  Will fill
in useful data once the infrastructure is inplace and properly tested.
Client, Tutor, Appointment specs.

Testing the infrastructure of these models.