Add :s3_host_name to Spree configuration options and update S3Support to...

In my project I'm using ```AmazonS3```, but images accessible only through region specific subdomain ```s3-eu-west-1```. Paperclip allows to setup such kind of domain using ```s3_host_name```, so I've added this key to ```Spree::Config``` to be configurable and updated ```Spree::Core::S3Support``` to use it.

P.S. I've tried to add tests for that, but looks like this changes could no be tested simply, because ```Spree::Config``` assing it's values on class loading.