Add missing __init__.py to formatters/ so setuptools.find_packages
includes the subpackage in wheel/sdist builds (#16).
Move ODTFormatter import from top-level to lazy import inside
get_formatter_by_extension() so missing odfpy no longer crashes module
load before main() runs. Remove dead except ImportError handler in
cli.py save() block; surface a clear 'odfpy is not installed' message
when the formatter factory returns None for .odt (#25).
Closes#16, closes#25
