Noramlize 'setup.py' to fit conventions:

- Comply with flake8, except for indentation width, which is 2.

- Move human-centric metadata to top of 'setup()' call.

- Add Trove classifiers for supported Python versions.

- Use 'find_packages()' + MANIFEST.in to avoid errors in listing
  modules and packages.

Closes #131 and #161.

python/MANIFEST.in

+prune google/protobuf/internal/import_test_package
+exclude google/protobuf/internal/*_pb2.py
+exclude google/protobuf/internal/*_test.py
+exclude google/protobuf/internal/*.proto
+exclude google/protobuf/internal/test_util.py
+
+recursive-exclude google *_test.py
+recursive-exclude google *_test.proto
+recursive-exclude google unittest*_pb2.py
+
+global-exclude *.dll
+global-exclude *.pyc
+global-exclude *.pyo
+global-exclude *.so