CdnClient: Cleanup CDN factory pattern
 * Change to static global factory similar to delivery network clients/exporters and persistent dictionaries
AdUnit Export: DeliveryNetwork content exporter cleanup
 * Cleanup delivery network content exporter to use improved CDN client factory
 * Add entity save to AdUnitExport activity
Merge branch 'MayRelease' of https://github.com/RareCrowds/OpenAdStack into MayRelease
ApiLayer: Fix null reference exception introduced in previous merge
AdUnit Export: Activity cleanup/tweaks
AdUnit Export: Try looking up publishers by code before creating them