Fully fledged command line interface

`datastringer` now exposes a command line interface with pluggable stringers.

A global install of datastringer would work as following:

```bash
npm install -g datastringer
```

The initial stringers have been moved into separate npm modules, which are required like `var stringer = require('uk-crime-stringer');` and still by exposing a unique function.

- [uk-crime-stringer](https://github.com/oncletom/uk-crime-stringer)
- [uk-local-police-stringer](https://github.com/oncletom/uk-local-police-stringer)

This way we isolate a bit more each stringer function, and it makes it easier to compose between public and private stringers while making their update easier.

The filesystem storage has been removed to isolate the app from the storage (and because Heroku deletes any file which is not versioned in Git).