Fix issue #166 - configurable threshold in objDisplay.

[romario333]

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/

[logicalparadox]

Perfect, thanks! I will take care of the documentation.