Make main camera globally accessible.

This commit makes the main scene camera globally accessible in a
sane manner by setting Entity.Camera.main to the first instance of
Entity.Camera that is created. Obviously, this may be overridden by
the user at any point if they wish to specify the main camera.
Implement interface component.

This commit adds the interface component which allows the user to
create DOM-based GUI elements anchored to the owner's screen position.
It also adds jQuery to the vendor libraries.