BUGFIX: contour.py - add_label_near()

`transform.inverted().transform([])` evoked an index out of bounds error:

```
/usr/local/lib/python2.7/dist-packages/matplotlib/transforms.pyc in transform_non_affine(self, points)
   2002             x_points = x.transform_non_affine(points)[:, 0:1]
   2003         else:
-> 2004             x_points = x.transform_non_affine(points[:, 0])
   2005             x_points = x_points.reshape((len(x_points), 1))
   2006 

IndexError: too many indices

```
calling `transform_point()` fixes the intended inverted transformation