Atom Shell Integration

This PR effectively merges all the former [Copay Shell](https://github.com/gordonwritescode/copay-shell) components into the Copay project.

You no longer need to clone a separate repository to develop and test the shell integration.

After merging, you'll need to:

```
~# npm install
~# npm run setup-shell
~# npm run shell
```

Documentation has been updated to reflect this.