Aggregations support

This PR adds support for aggregations framework added in elasticsearch 1.0.

This code will show average age by sex by country:

```go
	resp, err := conn.Search(query, []string{*index}, []string{})
	if err != nil {
		log.Fatal(err)
	}

	sex := resp.Aggregations["sex"]

	for _, sexBucket := range sex.Buckets() {
		log.Printf("sex: %f", sexBucket.Key())
		country := sexBucket.Aggregation("country")

		for _, countryBucket := range country.Buckets() {
			log.Printf("  country: %s", countryBucket.Key())

			age := countryBucket.Aggregation("age")

			log.Printf("    avg age: %f", age["value"])
		}
	}
```

I'm looking for feedback before adding tests :)