Moved attachment generation to report renderer

The ReportRenderer can be overridden using ScheduledReports.getRendererInstance event. This allows to render non-standard html/pdf/csv reports. If such report contains several graphs then it is not possible to send it as e-mail as the attachments are generated in ScheduledReports/API. This seems to be inconsistent. If a ReportRenderer can decide about the content of a report it should also be able to decide about the attachments.

I 've added small changes to the code which allow to override the default attachment creation process.