improve comment for hash_mixin
add file_id field to item solr docs
put whitelist filename in log
move solrj_wrapper mixin to a file in lib, and load it from record_merger
move nokogiri xml node and oai client monkey patches to files in lib, and load them in indexer.rb
move monkey patches into their own files in lib, and require them from appropriate classes; reorg remaining code a comments a bit