Rotate extent

Resolves #716
Added a rotation parameter to Polygon.configureExtent

example:         
polygon.configureExtent(new Cesium.Extent(
                Cesium.Math.toRadians(-120.0),
                Cesium.Math.toRadians(20.0),
                Cesium.Math.toRadians(-80.0),
                Cesium.Math.toRadians(50.0)),
                0.0, Cesium.Math.toRadians(20));

ExtentTessellator.computeVertices rotates the northwest corner counter-clockwise about the center of the extent, then calculates the positions of the other vertices based on that position.