Learn Python and Kana
check out repo, create a virtualenv, install requirements
git clone git@github.com:SteveClement/py-kana.git
cd py-kana
mkvirtualenv py-kana
pip install -r requirements/requirements.txt
You can use Hepburn or Kunrei shi-ki romanization.
There are other romanization schemes like Nihon-shiki more information on the Romanization of Japanese Wikipedia page.
pbRomanji.py - Check clipboard/pasteboard for CJK contents and print out the Romanization.