Allow protocol-relative URLs.

This helps you handle mixed-content errors when building URLs for external
sites. This should only be used when your site handles both http and https.

When the scheme is omitted, 'http' will continue to be the default when
building redirect URLs, since they're required to be fully qualified.
Add Joe Esposito to AUTHORS.
Update changelog with protocol-relative URLs.
Test the right thing.
Merge pull request #528 from joeyespo/protocol-relative-urls

Allow protocol-relative URLs