support diffs: add expected to AssertionError

cp. http://tjholowaychuk.tumblr.com/post/18574009869
support diffs: add actual to AssertionError
turn on diffs
SubstringMatcher: provide diffs
Extend diff support to `equalTo` and prevent `is` from breaking it.
Unfortunately, it doesn't make much sense to test `assertThat` using `assertThat`.
Update to latest mocha
Fix: "diff" code was inserted between "description" code.
Extend diff test to make sure that `actual` is provided
Implement diff support for `promiseThat` and clean up implementation in `assertThat`
Fix test that broke because errors are not described in more detail
Merge branch 'feature/error_json'
Merge branch 'pr7-alexbepple'
Update dist files
2.8.0