rational-numbers: implement exercise

[cmccandless]

There are exercises that are not yet implemented in Python track (the full list of them).
You can help us change this situation by implementing this one. Ref #417

The guide "Implement an exercise from specification" will help you succeed.

If you still have questions, I encourage you to ask them before starting working on the issue.

Please note that there are some special 'closing issue' keywords that you can put in description of your PR and Github will take care of closing the corresponding issue automatically after merge.
This simple thing helps us a lot, and makes our life a bit easier.

[mrcfps]

@cmccandless I'll work on this.