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 would move this chunk — `(this.type === 'path-group' ? this.height * vpt[3] : 0)` — to a separate variable, to make things a bit clearer.