Test SVG EXPORT

This is a preliminary work, nearly done for the svg export.
Final PR will have clean files, those commits are dirty.

@kangax if you can help me with the 2 failing tests, from polygon and polyline, i searche all fabric.js file and there is no trace of that error in case of missing points.
I even searched on past revision, but it looks like that function never existed.

I'm commenting some lines to explain you some decisions.
i don't think we need separate render function for the image.
it is similar to object one.