improve diff behavior, specially on white spaces and line breaks.

Change the diff to always use "Chars" diff since it will work better with white spaces.

Also make sure we replace invisible chars before doing the diff to avoid issues with leading line breaks just before end of the string.

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My changes doesn't affect the line numbers (so it's a better solution than #657)

See test that I added to `acceptance/diff`, try running it before/after my changes to see how it was improved. Here is a screenshot:

![mocha-diff](https://f.cloud.github.com/assets/155633/53608/ff9dda1c-5a6e-11e2-9717-c60a5df4a5d0.png)