improved documentation for Angle
added documentation for Matrix
added crossdot() to Vector

further changes:
- changed rotate()
- changed toString()
renamed Line.project() and Line.refelct()

new names:
Line.project() -> Line.projection()
Line.reflect() -> Line.reflection()
implemented methods in Polygon

methods:
Polygon.join(Polygon) <- wip
Polygon.centroid()
Polygon.nearestPoint(Vector)
toString()