#258 Adding Rotatable Billboards

[sstrandberg]

Adds rotatable billboards, which function identically to regular billboards with the exception of a settable up vector.

[sstrandberg]

[sstrandberg]

folds upVector functionality into main BillBoardSceneNode class instead of using a separate class.

[sstrandberg]

Changed it so that upVector functionality is in the main billboard class (toggleable, by default off) instead of using a separate class.