Respect cache headers from remote

[bertbaron]

Intravatar does currently not respect the cache headers from the remote services. The reason is that it allows caching for a short period (currently 5 minutes) for its own requests so that changes will be picked up in decent time. However, if a remote service generates images that should be cached for a longer period intravatar should respect this.
The only way seems to be to cache those requests ourselves, so it will require some effort to implement this (unless we can find some proxy library that does this already). On the other hand, caching might greatly improve response time and will offload the remote services.