Added json2csv - to parse tweets

Hi Robin, I've added a script to convert json files to csv. At the moment it just converts, but I intend to extend it to any analysis we want, running on the grid (distributed across cores). Otherwise the data are becoming a bit unmanageable!

 It will accept command-line arguments giving the fields to add to the csv, but it also has some defaults. E.g.:

python json2csv.py -o tweets.csv data/*

will take all data in 'data' dir and write to tweets.csv. Some fields don't work yet, I need to fix, but could you see if it works with your data? It seems OK on my ~70k collection, but before I change anything I want to see if it breaks down on a fuller set. It seems to handle some of the unicode characters that have been bothering you, at least on OS X (again this might have different behaviour on your OS).