Implement dashed stroke + some fixes with stroke, fill, removeShadow and clipTo + fixes in brushes

- fabric.Rect (now uses fabric.util.drawDashedLine), fabric.Line, fabric.Triangle, fabric.Polygon, fabric.Polyline, fabric.Image now supports dashed stroke
- fabric.Circle and fabric.Ellipse actual only supports native dashed stroke (browser must support context.setLineDash)
- fabric.Image.toSVG() has stroke/dashed-stroke support
- fabric.Object#clipTo has now affect on fabric.Image's stroke
- fabric.Shadow#affectStroke is considered - _removeShadow is now called in _renderFill (check whether stroke is to be affected) and _renderStroke
- reset shadow styles (contextTop) in fabric.PencilBrush, fabric.CircleBrush and fabric.SprayBrush (fabric.BaseBrush.removeShadowStyles)

Closes #603