Ability to specify a custom jspm_packages path to the bundle command

*Temporary PR/Code review until #69 is merged.*

We've ran into a situation where we want to create a bundle with jspm of a module included with jspm.

```
├── app
│   ├── app.js
│   ├── component.js
│   ├── dashboard.js
│   ├── inbox.js
│   ├── predashboard.js
│   ├── preinbox.js
│   └── router.js
├── config.js
├── jspm_packages
│   ├── es6-module-loader@0.7.1.js
│   ├── github
│   │   └── toranb
│   │       ├── toranb-react-jspm-view@0.2.2
│   │       │   ├── README.md
│   │       │   ├── bundle.js
│   │       │   ├── config.js
│   │       │   ├── final.js
│   │       │   ├── package.json
│   │       │   └── prefinal.js
│   │       ├── toranb-react-jspm-view@0.2.2.js
```

So say you jspm install `toranb-react-jspm-view@0.2.2` from your root "app" directory and you want to bundle that. Currently it doesn't work because it will look for `jspm_packages` in `toranb-react-jspm-view@0.2.2/jspm_packages`. Instead it needs to know that `jspm_packages` is actually up 4 directories.

This PR adds the ability to specify a custom jspm_packages path like so:

```bash
jspm bundle final bundle.js --packages ../../../../jspm_packages/
```

Simply added an argument to `setLocalPaths()` that allows for using a temporary jspm_packages path.

Is this something we could add to jspm after #69 is merged? @guybedford