Added some server-side support for authentication. Before we can do
any authentication we need to be able to represent important things in
the system such as users, courses, etc. We will also want to be able
to store this data in a database. This commit adds support for mongodb
connections using mongoose. I have created a server directory which
will include all server-side libraries and a server/models directory
which includes model definitions using mongoose. You can see two
examples of that for users (User.js) and courses (Course.js).