New exercise: python series
Merge pull request #10 from sjakobi/series

New exercise: python series
New exercise: python octal
Merge pull request #9 from sjakobi/octal
New exercise: python difference-of-squares
Merge pull request #11 from sjakobi/difference-of-squares
Rename difference-of-squares example implementation
New exercise: python luhn
Merge pull request #13 from sjakobi/luhn

New exercise: python luhn