Crawler

Currently, just has one useful function which returns the package_names for the top apps from each category. For some reason, the url only works if the 'start' is <= 199 and if 'num' is <= 120, so only grabs the top 320 paid and free from each category. I don't think we can currently download paid apps through the api though, just so you know.

Only really used in conjunction with some functions in database_helper to update the topApps collection and then scrape and add the new apps to the database.