Change of API in 0.7.1 with node.js

Hi!

In 0.7.0, Using `postal.diagnostics` was with something like this:

```
var DiagnosticsWireTap = require('postal.diagnostics')(_, postal)
```

With 0.7.1, this has became:

```
var DiagnosticsWireTap = require('postal.diagnostics')(_, postal)
```

For a minor update, this is quite disruptive.

`README.md` is not up-to-date with this change.
