Fix issue #166 - configurable threshold in objDisplay.

40-char limit after which expected and actual values are truncated is sometimes limiting. For example today I get this message:

    AssertionError: expected { Object (id, title) } to deeply equal { id: 1, title: 'pull-request 1' }

But here I would much rather get this message:

    AssertionError: expected { id: 1,
      title: 'pull-request 1',
      '$get': [Function],
      '$save': [Function],
      '$query': [Function],
      '$remove': [Function],
      '$delete': [Function] } to deeply equal { id: 1, title: 'pull-request 1' }

To solve this I have added `chai.Assertion.truncateThreshold` configuration property. It allows you to set threshold after which values should be truncated, or disable truncation altogether (if set to 0).