Fix issue #166 - configurable threshold in objDisplay.

This PR replaces PR #214.

Implementation notes:

* I have moved `includeStack` and `showDiff` properties to `config.js`. Original properties on `Assertion` are still there - they log deprecation warning and delegate to new properties at `config.js`.
* `config.js` is accessible either via `var config = require('config')` or `chai.config`
* Documentation should be changed (I don't know how):
  * Section Configuration here: http://chaijs.com/guide/styles/