OSMF SVN

Adding file missing from previous commit.

Adding HTMLGateway unit test, and applying more spec review changes (renaming IContainerGateway addChildElement and removeChildElement methods).

Add VAST media generator and tracking proxy. Implementation and unit tests complete for the former, partial for the latter.

Remove commented out line.

Committing HTML gateway feature code.

Removing obsolete import statement.

Removing obsolete import statements.

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.

Add ProxyElement for handling VAST Impressions.

Add missing override to unit test.