OSMF SVN

PARB feedback: Allow MediaElement subclass constructors to have default params. This involved moving the ILoader param to the second param, and providing a default when no loader is provided. For subclasses (like VideoElement) for which multiple loader types are supported, the implementation now chooses the appropriate one based on the resource type.

PARB changes: making "gateway" on MediaElement read-only.

Updating sample to the latest PARB changes (MediaPlayer.element to MediaPlayer.media).

PARB changes: renaming gateways to containers, renaming RegionGateway to MediaContainer, renaming HTMLGateway to HTMLMediaContainer. Renaming samples, and updating code and unit tests accordingly.