Deprecate AwsS3::getUrl() method and promote new gaufrette/extras repo
Deprecate AwsS3::getUrl() method and promote new gaufrette/extras repo

Since AwsS3 is the only adapter to have such a method, and because it's
a useful feature even outside of this adapter, I moved URL resolution
to the newly created Gaufrette/extras. It was partialy inspired by
K-Phoen/extras, so thanks to him.

NiR-/Gaufrette,KnpLabs/Gaufrette,KnpLabs/Gaufrette,NiR-/Gaufrette