Configuring Calling Format for S3 URLs

Debugging alleged connection issues with a secure site that uses S3 storage has revealed that in some instances, the default `SubdomainCallingFormat` used by `boto.s3.connection.S3Connection` for S3 bucket URLs can trip SSL certificate authentication in browsers, since wildcarded subdomains are being deprecated (http://tools.ietf.org/html/rfc6125#section-7.2). In some cases over HTTPS, this could trigger a browser warning, and Internet Explorer 10 on Windows 8 can even hide the issue behind an opaque "Connection Error."

For this reason, I believe the default (indeed, for `boto` as well) should be `boto.s3.connection.OrdinaryCallingFormat`.  I have therefore taken an opinionated approach here of using `OrdinaryCallingFormat` by default when calling `S3Connection` and allowing the user to configure the calling format otherwise using setting import string in `settings.URL_CALLING_FORMAT`.  For more details, please review the documentation update and the addition to `S3BotoStorage.__init__()`.