Lookup Engine update:

* Accept 'application/opensearchdescription+xml' mime type
 * Only use page icon when the OpenSearch description file is hosted at the same domain. This would make the interface less confusing if multiple search providers are listed on a single page (e.g. someone hosting a list of useful lookup engines on a single page)
 * Identify lookup engines by their source URL rather than name (sometimes title in <link ...> does not match name given in description file. E.g. PubMed)