Adds option to uninstall multiple packages at once.

It would be nice to be able to uninstall multiple packages at once:
`jspm uninstall p1 p2 p3`

If any of the packages are not primary installs, jspm fails with an appropriate warning. Otherwise all the packages are removed and cleaned.
This can just be `config.pjson.dependencies[name]`. The `installed` tree changes happen at the same time as `config.pjson.dependencies` or at least they should (install.js line 320).