Modify get id sites hook #157

This patch adds label of period object passed to ArchiveProcessor.Parameters.getIdSites hook so that plugin can recognize whether currently archived period is day or not-day, and accordingly to this replace idSite with array or not.

Thanks for suggestion. fixed in https://github.com/piwik/piwik/commit/d850fe794e5bb0de12f8dbb17b3acd56f1191e2d