Replaced some tabs with spaces
Added unittests.
Added some minor changes.
Added unittests for the calculate_coins problem.
Added unittests for the Cat problem.
Some minor change.
Added unittest for the cat2 problem.
Added unittests for the concat_files problem.
Added unittests for the contains_digit problem.
Added unittests for the contains_digits problem.
Some minor change.
Added unittests for the count_consonants problem.
Added unittests for the count_substrings problem.
Added unittests for the count_vowels problem.
Added unittests for the count_words problem.
Added the second file to the rm call in tearDown.
Added unittests for the generate_nimbers problem.
Added unittests for the goldbach problem.
Added unittests for the groupby problem.
Added unittests for the is_an_bn problem.