Support for Google App Engine

[pavelkraleu]

Backend tasks shouldn’t be redirected to SSL

[pavelkraleu]

These are App engine headers added only to backend (cron) tasks https://cloud.google.com/appengine/docs/python/taskqueue/overview-push?hl=en#task_request_headers https://cloud.google.com/appengine/docs/python/config/cron?hl=en#Python_app_yaml_Securing_URLs_for_cron

[mritchie712]

is this going to be merged? Seems like a straight forward change.

Edit - This works fine for my use case sslify = SSLify(app, skips=['mypath', 'anotherpath']) since my cron jobs only hit end points that aren't public

[Projjol-zz]

@pavelkraleu if you'd resolve the merge conflicts, I can go ahead and merge your changes, thanks 👍