add python rna-transcription exercise
prime_factors implementation passes first 9 tests, but is slow
speed up sieve
first ruby/prime_factors submission
slight refactor of ruby/prime_factors
update bob exercise
update ruby/scrabble-score
start ruby/clock challenge
implement addition method for ruby/clock
extract minute and hour into subclasses
refactor Clock.+
refactor and add Clock::MyNum.-
define Clock#==