Updated PKPoint

Moved PKNumber import to .m file
Removed NSCoding support for PKErrorBar

Removed NSCoding support for custom properties. Not sure  if they can
be used.
Changed layout with frame to bounds
Deleted PKPointMarker

There is a better way to do this.
Added PKErrorBarGroup

A group of error bars for the positive and negative, x and y directions.
Deleted PKShapeMarker

I have a better name for this class.