aggs: add percentiles rank aggregation

Add percentiles rank aggregation of Elasticsearch 1.3.0.
See
http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-aggregations-metrics-percentile-rank-aggregation.html#search-aggregations-metrics-percentile-rank-aggregation.

Fix documentation issue in percentiles aggregation.
add SearchSource; refactor search and highlight

* Add SearchSource (SearchSourceBuilder in Elasticsearch)
* Use SearchSource in SearchService to build search body
* Refactor Highlight to represent structure of ES 1.3.0
* Add FetchSourceContext
* Add Rescore and Rescorer
* Move SortInfo into its own file
aggs: add top_hits aggregation
aggs: add geo_bounds aggregation

Notice: Elasticsearch 1.3.0 has an error here in that it does not
output the aggregation name. So tests should fail with 1.3.0.
It is fixed in
https://github.com/elasticsearch/elasticsearch/issues/7004.
suggesters: add SuggestField; add test for completion suggester
update README and version; more tests for search source

* Update README.md to represent state of project
* Update version number to align with Elasticsearch
* Add more tests for search source