deal with float rounding error in Line.intersect(Line)

This commit fixes #41.

In the test case added in the previous commit, two non-intersecting lines
are very nearly collinear, but float rounding errors lead to incorrect
intersections reported.

This commit makes Line.intersect() treat denominators below 1e-9 as 0,
to make it more accepting of float rounding.

svgpathtools/path.py

@@ -572,7 +572,7 @@ class Line(object):
             d = (other_seg.start.imag, other_seg.end.imag)
             denom = ((a[1] - a[0])*(d[0] - d[1]) -
                      (b[1] - b[0])*(c[0] - c[1]))
-            if denom == 0:
+            if denom < 1e-9:
                 return []
             t1 = (c[0]*(b[0] - d[1]) -
                   c[1]*(b[0] - d[0]) -