showDiff: force true for equal and eql
lib: [Assertion] add configurable showDiff flag. #132
test: [Assertion] configruable showDiff flag. Closes #132
Merge branch 'feature/showDiff'

* feature/showDiff:
  test: [Assertion] configruable showDiff flag. Closes #132
  lib: [Assertion] add configurable showDiff flag. #132
  showDiff: force true for equal and eql

Conflicts:
	lib/chai/assertion.js
	test/display/message.js