Abstract backend for pluggable OP

[lovato]

GAE does not accept any python package that needs to be compiled. And Flask-Images meets that requirement.

But F-I uses File I/O, which is not allowed inside GAE.

I can help with this one, but need to know first if you are already working in something like this... Or what would be the best approach. Get tied to some GAE offering (like https://developers.google.com/appengine/docs/java/googlecloudstorageclient/) or make it all-in-memory.

Best Marco