Elo-ranking

Fixes #13, historical ELO-rating.

Rating is still naive, on /api/scoreboard all ratings are calculated from beginning of time.

Changes database model, thus must migrate data when this change is applied.

Refactored quite alot of code.

New DB-model:
```javascript
{
	"_id" : ObjectId("5474dec229247100009214b2"),
	"date" : ISODate("2014-11-25T19:55:46.362Z"),
	"loser" : {
		"for" : 3,
		"against" : 10,
		"players" : [
			{
				"name" : "olav"
			},
			{
				"name" : "pål"
			}
		]
	},
	"winner" : {
		"for" : 10,
		"against" : 3,
		"players" : [
			{
				"name" : "calle"
			},
			{
				"name" : "stien"
			}
		]
	},
	"__v" : 0
}
```