Some minor edits to the loops lesson

I added a challenge problem that introduces while loops--these don't seem to be critical to the lesson material but they are an important element of programming and it seems useful to at least mention them here. I also reworded some of the explanation/discussion of mutable vs. immutable variables, in an effort to improve clarity. Finally, I made a couple of other (very minor) changes to some examples.
This is a good explanation, but be sure to also change the definition [in the glossary](http://software-carpentry.org/v5/gloss.html#mutable) for consistency.