Add signed_url method to AWS::S3::File and tests

N.B. this builds upon changes from PR #6 and #8.

This returns the signed URL for public access to a file, the expiry time can be passed into the method or will default to now + 1 hour in the future. Add perldoc and tests for these changes, update MANIFEST.

Although the URL is built on the client side this has been implemented using the same structure as other requests with a GetPreSignedUrl class that uses the AWS::S3:Roles::Request role to get at the _uri attribute. We don't actually go out to AWS for this method, but who knows what the future holds?