[metadata] Flesch–Kincaid readability test

[sethwoodworth]

http://en.wikipedia.org/wiki/Flesch%E2%80%93Kincaid_readability_test#Flesch.E2.80.93Kincaid_Grade_Level

Parse each english(?) text file with NLTK and get the Fleasch-Kincaid readability test of each book. Put that data in metadata.json, and change the README updater to add it to the README if the value is present in metadata.json.