Support generating validation report

[jwhitlock]

Google provides an open source feed validator:

https://code.google.com/p/googletransitdatafeed/wiki/FeedValidator

It is not easy to call the code from django-multi-gtfs (it is built around loading the data from a zip file), but it should be easy to write a task that export the feed to a temp file, runs the feed validator as a subprocess, and imports the resulting HTML into the database. This would simplify my own GTFS export process.