Added simple doctest examples for most of the lenses

The examples here are pretty simplistic. Mostly it's just examples of how to get data out of a string of json.  Hopefully I can expand them out a bit to show more interoperation with the core lens functions.  Let me know if I should target them more or less advanced from where they are.

Question:  What is a good spot for a larger example?  One that reads in a medium sized json string (a web service api response for instance), and shows how to traverse and edit it? I'm new to haskell and haven't seen a good spot for package-level docs.  README of course would work, but that wouldn't show up on hackage.