added method to represent HalfEdge vector start --> end
added method to get normals...

there seems to be a bug that causes to get the faceNormal to be (0,0,0)
which leads to (NaN,NaN,NaN) when it's getting normalized. Watch at the
toVec() method in HalfEdge.java - the edge 1-->0 is (0,0,0) which is
obviously not correct.