Add requirements.txt and ignore virtualenv directory

I typically create a virtualenv directory in my working directory under
the name “env”, so I can git pull the repo, go into that directory, do
“virtualenv env”, then env/bin/pip install -r requirements.txt to get
going (or just activate the virutalenv).
Merge pull request #2 from cjs/add_requirements

Add requirements.txt and ignore virtualenv directory