Improve efficiency by reducing max diameter on Python 3

[tomaz-suller]

A few small modifications:

• Fix syntax to allow code to run on Python 3 (and still run on Python 2)
• Imrpove circle selection heuristic: each time a circle candidate is rejected, the current max diameter is reduced by one unit until it reaches the min diameter so next circles are more likely to fit in the image
• Also add example image to repo so the script can be called without any command line arguments.

[tomaz-suller]

@franciscouzo could you take a look at this?