Add operating system trove classifiers

Resolves #4.

Author: ShaneHarvey

setup.py

@@ -57,6 +57,9 @@
         "Development Status :: 4 - Beta",
         "Intended Audience :: Developers",
         "License :: OSI Approved :: Apache Software License",
+        "Operating System :: MacOS :: MacOS X",
+        "Operating System :: Microsoft :: Windows",
+        "Operating System :: POSIX",
         "Programming Language :: Python :: 2",
         "Programming Language :: Python :: 2.6",
         "Programming Language :: Python :: 2.7",