Image format retrieval support

I have prepared a basic format retrieval support for `image_voodoo`. Hope you like it.

It fixes #5 PR.

It's compatible with AWT & GAE. 

Usage from commandline: 

    $ bin/image_voodoo --format samples/checkerboard.jpg
    JPEG

or in sourcecode: 

    p ImageVoodoo.with_image('test/pix.png').format
    "PNG"

Fortunately my editor was removing trailing whitespace so that is commited in single change-set.

When merged it will close PR #5.