add license info to metadata (#260)

Author: aaronjheng

setup.py

@@ -112,6 +112,7 @@ def __init__(self, *args, **kwargs):
       description=desc,
       long_description=long_desc,
       url='http://msgpack.org/',
+      license='Apache 2.0',
       classifiers=[
           'Programming Language :: Python :: 2',
           'Programming Language :: Python :: 2.7',