add npm publish script

This should make it easier for us to publish updates to npm

here's a pseudocode shell script:

```
# bump version in package.json, following semver
# do this with sed or some similiar 
# unix text editing scripting
VERSION=v0.12.2
git add package.json
git status
git commit -m "bump version to $VERSION"
git push

git tag $VERSION
git push origin --tags

npm publish
```