stats endpoints

- some minor refactoring
- added endpoint for multiple summoner lookup
- added endpoints for stats

Multiple Summoners:
```
Vigor.summoners([31640242, 31570201])
 => [#<Vigor::Summoner:0x000000015f2250 @fields={:name=>"Idea"}, @id=31640242>, #<Vigor::Summoner:0x000000015f20e8 @fields={:name=>"LEVEL 60 MAGE"}, @id=31570201>] 
```

Stats:
```
2.0.0p247 :004 > idea = Vigor.summoner("idea")
2.0.0p247 :005 > stats = idea.stats
2.0.0p247 :007 > stats["RankedSolo5x5"]
 => #<Vigor::StatsSet:0x00000000eb22d8 @losses=72, @modify_date=#<DateTime: 2013-12-30T00:18:50+00:00 ((2456657j,1130s,0n),+0s,2299161j)>, @wins=87, @stats={"totalChampionKills"=>581, "totalMinionKills"=>9847, "totalTurretsKilled"=>86, "totalNeutralMinionsKilled"=>2400, "totalAssists"=>1671}, @type="RankedSolo5x5">
```

Champion Stats:
```
stats = idea.champion_stats
taric = stats["taric"]
taric["totalAssists"]
 => 51 
```

Give me a day to sit on this, I'm not 100% happy with how the stats classes are structured, so I might wake up with some better ideas.