update wp-pubmed-reflist to version 0.6. Improved flexibility and building composite queries

git-svn-id: https://plugins.svn.wordpress.org/wp-pubmed-reflist/trunk@928314 b8457f37-d9ea-0310-8a92-e5e31aec5664
update wp-pubmed-reflist to version 0.6. Fix stable tag in reaadme.txt

git-svn-id: https://plugins.svn.wordpress.org/wp-pubmed-reflist/trunk@928315 b8457f37-d9ea-0310-8a92-e5e31aec5664