Document that PyYAML is implemented with Cython (#244)

diff --git a/setup.py b/setup.py
index f887975..a1a2baa 100644
--- a/setup.py
+++ b/setup.py
@@ -25,6 +25,7 @@
     "Intended Audience :: Developers",
     "License :: OSI Approved :: MIT License",
     "Operating System :: OS Independent",
+    "Programming Language :: Cython",
     "Programming Language :: Python",
     "Programming Language :: Python :: 2",
     "Programming Language :: Python :: 2.7",