Remove the `testing_creds.json` requirement

We forgot to do this after stubbing the test suite. This also updates
the LICENSE year and the testing command.
Add the bin npm field and document current usage

This is *far* from the interface I actually want. The `"bin"` field is
set for easier installation, before `dit` is published. People may now
install with:
```bash
git clone https://github.com/yamadapc/dit.git &&
cd dit &&
npm install -g .
```