Add scripts for release to prebuild, clean.
Add scripts for release to prebuild, clean.

The prebuild script uses pandoc to automatically convert the README.md
file into a README.rst, so that it can be rendered correctly on pypi.
The clean script simply cleans the build artifacts from the directory.
To use:

    python setup.py prebuild

And, after using twine to release,

    python setup.py clean

terrencepreilly/darglint,terrencepreilly/darglint