Convert solver functions into a solver class.

This is prep for having additional solvers that can be trivially
swapped in and out.

test_solver.py

@@ -6,9 +6,11 @@ import solver
 
 
 class TestSolver(unittest.TestCase):
-    def test_scored_words(self):
+    def test_SimilaritySolver(self):
         """Minimal end to end test."""
-        result = solver.scored_words()
+
+        solve = solver.SimilaritySolver()
+        result = solve.scored_words()
 
         expected = [
             ("cares", 139631),