Add hamming problem

I recently realised that these test cases aren't good enough. I've changed them in Ruby:
```ruby
assert_equal 1, Hamming.compute('AGAGACTTA', 'AAA')
assert_equal 2, Hamming.compute('AGG', 'AAAACTGACCCACCCCAGG')
```

This makes sure that there's actually a difference, and that it's a different hamming distance for the two cases. It avoids some n00b shortcuts.