consistent naming for spawn helpers
saner example for setting postinstall
handle optional from package.json instead of commandline flags
use pypi+semver to determine downloadable versions of virtualenv
handle errors removing the old virtualenv home
updated documentation for using requirements.txt and advanced usage
use requirements.txt instead of deps in package.json
make installation output more similar to virtualenv command output
better handling of metadata
rebuild should happen when requirements.txt changes
propagate temp dir removal errors
only re-download virtualenv when necessary
support doing simple pip updates
version 0.1.0