Refactoring.

Moved some of the functions around to separate scoring functions
from helper functions.
Test.

Added a function and a test for pairs. If a pair is found in the
hand it should return the score of the cards of that pair.
Implementation.

Added an implementation for pair-p. It sorts the hand based on
card score and then walks down the list returning as soon as it
finds two of the same scores.
Test.

Added another scoring function, two-pair-p, and a simple test
for it.
Implementation.

Added a quite complicated and cumbersome implementation for two-
pair-p. I really wanted to build on top of pair-p (which I've
done) but the result is not very pretty.

I think now would be a good time to go through the code and
refactor both pair-p and two-pair-p.