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.
Ok, as you wish. 
As I said, I am using it on CentOS 6.x, which has installed 2.6 python and it works perfectly... 
But I understand your point, since 2.7 is born on 2011 ;)