Implement S3 deletion

* Wrapping S3 API's delete_object method
* Update tests - change to the {bucket}.{region} format requested by S3
* Add test coverage for delete method - check that when a file is removed from S3 it subsequently 403s

Note that to run the tests I replaced $configuration with my own bucket configuration.
Add exists method

It can be useful to check if a file exists on S3