MANIFEST.in: Distribute the examples/ directory

Avoid:

  $ make -C docs html
  ...
  /.../Flask-Cors-1.7.1/docs/index.rst:121: WARNING: Include file u'/tmp/Flask-Cors-1.7.1/docs/../examples/simple_example.py' not found or reading it failed
  ...

by packaging the referenced files.
Merge pull request #50 from wking/distribute-examples

MANIFEST.in: Distribute the examples/ directory