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 we can make this function allow names to be a string as well with a simple check:

```javascript
if (!(names instanceof Array))
  names = [names];
```

Will be useful for exposing uninstall in `api.js`.