refactor remove module, fixed #186

1. list all the modules, find which versions to be removed and which to
be remained.
2. remove all the modules need to be removed
3. list all the tags, find which tags need to be removed and remove
them.
4. if the latest tag removed, need to generate a new latest tag.
fix if delete all the versions
Merge pull request #187 from cnpm/issue186-remvoe

refactor remove module, fixed #186