Create growingrecs.py

Command-line tool to find the most common planting conditions for a given crop. This version counts combinations of planted_from and sunniness, but as planted_from is often left blank this may not be the best choice.
Incidentally, Growstuff's own code for how we generate that is in app/models/crop.rb in the methods crop.sunniness and crop.planted_from, which each return a hash. The actual display of this data is in app/views/crops/_planting_advice.html.haml