Comment 11 for bug 907056

Pushed another commit onto the pile. This one includes library ranking for holdings, specifically volumes and copies, such that copies will be returned in ordered preference as follows:
  1. circ_lib matches the (new parameter) preferred library;
  2. circ_lib matches the search_library or its descendants, in order of depth
  3. All other copies in scope but not matching either the circ_lib or the search_library/descendants are ordered by library name

This also creates new function, evergreen.rank_ou(), for the purpose of generating a ranking value for the copies.