Implemented champions methods.

Implemented methods to access the `/champions` endpoint of the Riot API.

This includes tests.

What Faraday returns for the call is a bit different from the rest of the API calls.  It returns a Hashie of the form:

```
> response 
=> #<Hashie::Rash champions=[#<Hashie::Rash active=true attack_rank=8 bot_enabled=false bot_mm_enabled=false defense_rank=4 difficulty_rank=6 free_to_play=false id=266 magic_rank=3 name="Aatrox" ranked_play_enabled=true>,...
```

thus the call to `.champions` in `list`.