Add digits support for pyocr (Tesseract only).

As per #17, this is the pull request for digit only tesseract OCR.

I updated test cases by adding an image to scan for digits.

Also I lowered the requirements to Python 2.6 because I successfully used pyocr on a 2.6 environment.
I'm not going to support and test with Python 2.6, so I can't offer this guarantee. :/