Update Bio::PubMed to use the new end-point

Looks like the PubMed endpoint has changed. The '/entrez' end point now does a permanent redirect to '/pubmed' for all requests. I've updated Bio::Pubmed to reflect this.

I've also updated #query so that it correctly parses the results returned by the current end point, and handles multiple ids correctly.

I've also removed the error check, which would never work as it attempted to regex an array. In addition, PubMed simply returns no result if there are no matches for an id, rather than an error message.

I've added a test in the network area for Bio::PubMed#query.