* Allow list of entity ids to be loaded to better support idpList

Certain metadata sources, like mdq, were don't support loading all metadata which
prevented them from being used with idplist. This change adds support for allowing
metadata sources to load a list of entities, allowing MDQ to be used for idp list.

bugfix: Make sure the PDO and Serialize metadata storage handlers return metadata containing the 'entityid' key.

While investigating issue #393, we noticed that these two handlers don't enforce the entity ID of each entity to be set in the 'entityid' key of the metadata array (the Flatfile and XML handlers to enforce this). Since this is the way we propagate the entity ID (code using metadata won't necessarily have access to the key of the metadata array containing the entity ID, or such a thing might not even exist), we need to make sure the entity ID is set inside the array. Otherwise, if the metadata for an entity does not contain it directly, the 'ExpectedIssuer' would be set to null, an error recorded, and authentication would fail as the entity ID of the IdP issuing an assertion would not match.

(being respectful with occurences that might change the behaviour, i.e. default database prefixes)

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1552 44740490-163a-0410-bde0-09ae8108e29a

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1520 44740490-163a-0410-bde0-09ae8108e29a

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1451 44740490-163a-0410-bde0-09ae8108e29a