add version file, bump version

[chadawagner]

I'm not sure what the official release process is for this repo, but it's been nearly 2 years since the last release, and 3 months since the latest merge of changes that someone might want to use. Unfortunately it can't just be installed from git because setup.py fails due to a missing version.txt file. This PR just adds that file and removes it from .gitignore so it can be installed without a PyPI release, and bumps the version