issue #135

added a map attribute to the ReportingPosition, and adapted the
ReportingArbeitsauftrag to generate maps for each position.

new configurable attributes were added too.
POSITIONMAP_URL
POSITIONMAP_WIDTH
POSITIONMAP_HEIGHT
POSITIONMAP_BUFFER
the old MAP_* attributes where refactored to OVERVIEWMAP_*

the report creation process is now a implemented as an AbstractCancellableDownload
Merge branch 'feature/135-arbeitsauftragreport-multiplemap' into dev