Add message to failed test for string comparisons that shows expected and actual Strings.

Example:
--- Expected +++ Actual @@ @@ -'string' +'different string'

![screen shot 2013-05-01 at 9 15 00 am](https://f.cloud.github.com/assets/1474660/449542/cf8729c8-b275-11e2-871a-efffa9b53d35.png)