Move data

I moved all of the data files to the top-level 'data' directory and added a README with a standard set of information for each set of files. It would be great if @jennybc @apawlik @cbahlai @benmarwick can review the README to make sure I've described the data sets correctly. Also made changes to some of the lessons that referred to data files in a different location, but I did not go through all of them in detail. I will make a separate issue for updating the lessons.