Added shoulda to gemfile to make validation testing easier
Added friendships table with user_id and friend_id
Added index on user id to runs table in DB
Added validation for user_id on friendships
Added validation for friend id on friendships
Added and tested for a friendship belonging to a user
Added belongs to a friend relationship for user and designated class as User to ensure a friend is actually a User
Created has many friends, inverse friendships, and inverse friends for users
Incorporated shoulda-matchers tests into run spec
Fixed merge conflicts