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

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/1474660/449462/3a1a4822-b273-11e2-8bc2-3b799f64dbea.png)