Change of API in 0.7.1 with node.js

[vincentbernat]

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.