Declare package classifiers.

It is declares python2-only at the momens since there are still issues with
python3 support.

Related:
 - github pull request #22
 - github pull request #26
 - github pull request #29
 - github issue #28
 - github issue #20

	Change on 2016/06/20 by vrusinov <[email protected]>

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=125322335

Author: vrusinov

setup.py

@@ -40,4 +40,12 @@
       url='https://github.com/google/python-gflags',
       packages=['gflags', 'gflags.third_party', 'gflags.third_party.pep257'],
       data_files=[('bin', ['gflags2man.py'])],
+      classifiers=[
+          'Programming Language :: Python :: 2 :: Only',
+          'Programming Language :: Python :: 2.7',
+          'Intended Audience :: Developers',
+          'Topic :: Software Development :: Libraries :: Python Modules',
+          'License :: OSI Approved :: BSD License',
+          'Operating System :: OS Independent',
+      ]
      )