Normalize Results entities to be EntityList instances

PubMedSearchResults and SearchResults have been removed, and in place of them
PubMedIdList and SearchResultList have been created. This way, all entity
listings are normalized to be of type EntityList.
Interface for PubMed Provider

PubMed Provider web interface has been implemented inside the download
documents section (in the administration panel). It can download a list of
PubMed IDs given as comma-separated values and search in the PubMed database
for some PMIDs to add to the list.
Add MIT License