Allow searching for specific media type independently of the lane's media type

[leonardr]

The admin interface uses the standard /search endpoint to search a library's top-level lane. Currently this only searches ebooks. A different issue will cover changing the top-level lane to include all types of media.

The search controller needs to be changed further to allow the caller to pass in a specific type of media it wants to search. This goes beyond what OpenSearch allows for, but it's not a big deal and doesn't open up the full "advanced search" can of worms, because the URLs will only be used in the admin interface and in direct links where the context will be clear to the user.

The client should identify the media type it wants to search for using the schema.org URIs we've adopted: http://bib.schema.org/Audiobook and http://schema.org/EBook. If no media type is specified, all media available from the selected lane will be searched.