Implement methods to hit the league endpoint

You should pull https://github.com/petems/riot_api/pull/11 first.

Froggen has an awful lot of league data. :frog: 

This feature, I think, underscores the need to implement classes instead of using Hashie.  I found some weirdness when testing the feature in irb:

```
>> entries["entries"].first
=> #<Hashie::Rash is_fresh_blood=false is_hot_streak=false is_inactive=false is_veteran=false last_played=0 league_name="Tryndamere's Inquisitors" league_points=74 losses=0 player_or_team_id="25301501" player_or_team_name="Tovvel" queue_type="RANKED_SOLO_5x5" rank="V" tier="DIAMOND" time_until_decay=-1 wins=333>

>> entries.entries.first
=> ["timestamp", 1386868370660]
```

Where all other API call results I could call a method on the Hashie result, here I have to use Array notation.  I have no idea why, I've never used Hashie that much.  And b/c I've never used Hashie that much I don't know how hard/easy it'll be to implement classes.  I've done a little work with Virtus (https://github.com/solnic/virtus) so hopefully they are similar.