Prototype `serve` command.

The server could potentially be used to:

- display published documents
- reserve new item IDs
- provide REST interfaces other viewers or clients


Some scenarios:

```bash
$ doorstop serve
serving at: 1.2.3.4:5678
```

```bash
$ doorstop serve --port 9999
serving at: 1.2.3.4:9999
```

```bash
$ doorstop serve --hidden
serving at: 1.2.3.4:5678  # only necessary functional interfaces, no items visible
```

```bash
$ doorstop serve --no-pull
serving at: 1.2.3.4:5678  # items will not be pulled from the remote repository
```

TODO:

- [ ] prototype REST APIs
    - [x] make everything return a dictionary
- [ ] wait for #84 
- [ ] select a default port: http://serverfault.com/questions/610058
- [ ] re-enable CI