Fix typo in encodings stubs

Fixes #369.
Fix importing mypy.codec.pytokenize in Python 3

Closes #370.
Move mypy/codec/example.py to samples/codec/example.py

The custom coding was causing all kinds of trouble during installation.
made access to data like vm, stubs, and lib portable
switch to entry point and incorrect extension on stubs
added codec line to fix encoding issues
fix package_dir declaration and typing modules -> packages