Update README.md

Fix virtualenv/pip instructions, and add note about making GnuCash API
visible to the virtualenv
Add more debugging code

To help resolve a GnuCash issue:
http://lists.gnucash.org/pipermail/gnucash-user/2013-February/thread.html#48012
http://lists.gnucash.org/pipermail/gnucash-devel/2013-February/thread.html#34990