OpenSearch support

[mpgirro]

The OpenSearch response elements are used in feeds for search-related metadata.

Three elements of the standard are encountered in feeds:

• opensearch:totalResults
• opensearch:startIndex
• opensearch:itemsPerPage

There are actually two versions of the standard and thus also two namespaces in use (elements have the same occurence frequencies in the latest analysis results):

• 4,2% declare http://a9.com/-/spec/opensearchrss/1.0/
• 0,1% declare http://a9.com/-/spec/opensearch/1.1/

The standard also defines a link relation rel="search", relevant for #85, as well as a media type application/opensearchdescription+xml we can provide as a constant in our MediaType implementation.