Adds Game class and RSpec tests:

1. Game class exists
2. Game class receives a unique game ID

Game class takes two player IDs and a match ID
Added a third parameter to Game initialize method: match ID
Adds Invitation class and initialize method. Initializes with host_id and guest_id. Invitation class 'exists'.
Added a unique ID for each invitation instance and a test to check
Removes the player ID parameters in Game initialize class. Game instances will be associated with Matches, which will take player IDs.
Merge branch 'felguerez'