OSMF SVN

Applying HTML support prerequisite refactorings:

* Making 'gateway' of type IMediaGateway a property on media element.
* Adding a GatewayChangeEvent, removing RegionChangeEvent.
* Removing RegionTargetFacet and setRegionTarget utility function.
* Adding IMediaGateway, and IContainerGateway interfaces.
* Adding a 'gateways' package, and moving RegionSprite into it. Removing the 'regions' package.
* Updating framework client code, unit tests and sample code accordingly.

Adding a sample that demo's using nested Regions.