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.
Perhaps allow the uninstall to go ahead for the other dependencies anyway? Saves the user having to type again?