Fix wrong positioned bounding box of fabric.Polygon and fabric.Polyline objects

- Substract minX and minY from points.x/points.y (_calcDimensions)
- Same in fromElement - but only if minX or minY is negative

Fixes issue #147. 

I have tested the polygons/polylines from http://fabricjs.com/test/functional/ and also the jsfiddle posted in issue #229 (http://jsfiddle.net/r2ZE7/11/).