[Adding a friend feature]

- Feature added: Adding a new friend to an existing game
- Friend managment spec
- Send an email to the friend when its added
- Email data configured using env vars
- Capybara-email support for specs
- Game Invitation model added (links User with games)
[Adding a friend feature - refactoring]
- Virtus gem incorporated.
- InvitationManager virtus model added to handle the game invitation logic.
- InvitationManager model spec.
- Friends management spec refactor.
Merge pull request #10 from insignia4u/friends2

Friends2