cleanup of aggregations api
- add javadocs
- remove Iterable from all multi-bucket aggregations
- all single-bucket aggregations should have getDocCount() and getAggregations()
- all multi-bucket aggregations should have getBuckets() that returns Collection
- every multi-bucket aggregation should have these methods:
 - getBuckets() : Collection
 - getBucketByKey(String) : Bucket
 - getBucketByKey(Number) : Bucket (only for numeric buckets)
 - getBucketByKey(DateTime) : Bucket (only for date buckets)
 - getBucketByKey(GeoPoint) : Bucket (only for geohash_grid)
- every bucket in all multi-bucket aggregations should have these methods:
 - getKey() : String
 - getKeyAsText() : Text
 - getKeyAsNumber() : Number (if the key can be numeric value, eg. range & histograms)
 - getKeyAsGeoPoint() : GeoPoint (in case of the geohash_grid agg)

 Closes #4922