Add support for OpenSearch

Added support for **Open Search**.

It is used to describe a search engine.

* [OpenSearch specs] http://www.opensearch.org/Specifications/OpenSearch/1.1
* [OpenSearch wiki] http://en.wikipedia.org/wiki/OpenSearch
Use the new Ruby 1.9 hash syntax.<br>Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.