Path.area() fails if it includes Arc objects

[SebKuzminsky]

Path.area() calls poly() on each segment, but Arc() doesn't provide that method. The area can be approximated using linear interpolation of the Arc:

def approximate_path_area(path):

    """Approximates the path area by converting each Arc to 1,000
    Lines."""

    assert(path.isclosed())
    tmp = svgpathtools.path.Path()
    for seg in path:
        if type(seg) == svgpathtools.path.Arc:
            for i in range(0, 1000):
                t0 = i/1000.0
                t1 = (i+1)/1000.0
                l = svgpathtools.path.Line(start=seg.point(t0), end=seg.point(t1))
                tmp.append(l)
        else:
            tmp.append(seg)
    return tmp.area()

[mathandy]

OK, I'm cool with adding this. Make a pull-request and I'll merge it.