oai v0.4.0

NEW FEATURES

• the requests are now made using httr::RETRY() rather than httr::GET() to
  facilitate retrying with delay control etc. Consult further the documentation
  of 'httr' package (#64)
• @mbojan takes over package maintenance from @sckott

MINOR IMPROVEMENTS

• update packages test to catch the 'cannotDisseminateFormat' OAI-PMH-level
  error correctly
• added package hex logo