Fixed use of option "expose files".

Hi,

For METS and omeka-xml, the option ```oaipmh_repository_expose_files``` is not checked and files are always added. This is fixed, except for omeka-xml format. Other changes are just for similarity. See https://github.com/zerocrates/OaiPmhRepository/issues/8#issuecomment-59038748.

Sincerely,

Daniel Berthereau
Infodoc & Knowledge management