Added Rakefile and task to run all problems solutions as tests
Added Graph and Node classed for graph-related problems. Added not finished yet solution for problem 18.
Commented output instructions to make rake test output looks more clean.
Solved problem #21. Modified Fixnum class to store prime_numbers (up to 10^6) as class instance variable.
Added file with graph data for problem #18.
Inject replaced with reduce in a few places.
Solver problem #22.
Solver problem #23.