Distribution: Add MANIFEST.in

`yamllint/conf/*.yml` remains in `setup.py`'s `package_data` because it
needs to be installed when running `pip install .`.

`docs/*` and `tests/yaml-1.2-spec-examples/*` just need to be packaged,
they can go in the manifest.

setup.py

@@ -44,8 +44,7 @@ setup(
 
     packages=find_packages(),
     entry_points={'console_scripts': ['yamllint=yamllint.cli:run']},
-    package_data={'yamllint': ['conf/*.yml'],
-                  'tests': ['yaml-1.2-spec-examples/*']},
+    package_data={'yamllint': ['conf/*.yml']},
     install_requires=['pyyaml'],
     tests_require=['nose'],
     test_suite='nose.collector',