Implemented the showing of a Diff for differing strings, like Rspec does.

https://github.com/garybernhardt/expecter/issues/3

Works for strings and unicode.

Points of attention:
- Might break in Python 3 I have only tested it in 2.7
- Now that I read the issue again, perhaps I should test for newlines, right now a diff is shown for all failing str's and unicode's, not just long ones.

Let me know what you think.