implement MPD#where, which allows complex queries.

It replaces #search, and matches more to other ORM's.
Rename :case_sensitive to :strict to better describe it's effect.

Docs were updated to match the new methods.
MPD#song_priority: accept multiple ranges or ID's
use #where instead of #search
Similar to MPD#where, replace #queue_search with #queue_where.

Apparently undocumented, but MPD supports multiple search terms
in "playlistfind" and "playlistsearch" too.
Fix some mistakes in the README