Adds ability to add a friend

I've added a `#add_friend` method to the client - it just takes a parameter of a username and then adds them. Naturally, I've written specs and tested it.

I've also taken the liberty to add Rspec to the development dependencies in the gemspec, since this wouldn't work for me otherwise since I don't have Rspec as a global gem.

PS - Awesome work on this gem. I'll be sure to share what I'm working on once it's done :+1: