Add virtual tab placeholder for new Product/Content editing

* Re-create the IContent->ContentItemDisplay map so we can add a custom AfterMap event (probably a hack?)
* Check for existence of merchProductKey property before adding tab
* Add virtual tab and property (placeholder for now)
Merge pull request #623 from rustyswayne/1.1.1

Adds NotificationMessage methods to NotificationGatewayApiController
Merge pull request #597 from Merchello/feature/content-tab

Product Editing in Content
Merge remote-tracking branch 'merchello/1.2.0' into 1.1.1

Conflicts:
	src/Merchello.Web/AutoMapperMappings.cs