Adding friends - first cut

This gives us the friends subcommand:

```shell
› rdio help friends
NAME
    friends - Manage your friends

SYNOPSIS
    rdio [global options] friends [command options]  add

COMMANDS
    add - Add a friend

› rdio friends add pengwynn
You are now friends with pengwynn.
```

There's a lot more to do, but this gets us started. I made a few changes to the specs; the `#any_instance` mocking was giving me some trouble.