Reversed type detection of the 'image' parameter

Hi, thanks for your work!

The type detection of the 'image' parameter in ``triangulize()`` caused some problems for me.

* testing for isinstance Image.Image only worked if PIL was imported using "import Image" but not when using "import PIL.Image"
* I'd like to pass in objects of type Image._ImageCrop.

That's why I reversed the type checks to see if 'image' is either a string or a file-like object ('read' property).

If you like it, please merge!