Adapt to ElasticSearch v1.2.0 + Bump to 2.1.0

Based on work by @laureek.

Use SearchContext.close() instead of SearchContext.clearAndRelease().

Use IndicesOptions.lenientExpandOpen() instead of deprecated name
IndicesOptions.lenient().

Using RestBuilderListener instead of bare ActionListener, and
BytesRestResponse instead of XContentRestResponse.

Closes #15.
Closes #19.