Add support for Users to the backend

[cjhammons]

To support users we need the following:

A users table in the database with the schema:

• id: primary key
• username
• hash_password: the user's password encrypted
• avatar_path: The filepath of the user's optional avatar

A user struct:

type User struct{
     ID int
     UserName string
     HashPassword string
     AvatarPath string
}

And finally, standard CRUD opeartions in db.go