Added SimpleDiff

I often have instances where the diff printout obstructs the problem rather than clarifying it.  For instance:

![Screen shot 2013-01-15 at 5 08 30 PM](https://f.cloud.github.com/assets/312943/88553/5276eae8-64f2-11e2-8018-b2b5a794d9c1.png)

With the change I made, it now avoids the work of interspersing the actual/expected:

![Screen shot 2013-01-22 at 5 09 54 PM](https://f.cloud.github.com/assets/312943/88554/61a3f2f4-64f2-11e2-8b82-7e2d4239f439.png)

I suppose I don't expect this to be accepted vanilla, but maybe we could have something which detects how many characters (or characters in a row) are different.  If there's a big difference (easy to see, likely completely different strings), just show them side by side with simpleDiff.  If there's a small difference (hard to see, probably a similar string), then use your existing diff.

Related to this, it seems like the showDiff flag is being ignored.  I'm using should.js, and upon removing the showDiff flag there, mocha still displayed the exact same output.  Is that intentional?

It sure would be nice to have a config option for diff formatting!