New Feature: Protocol support

### New Features:
* Adds protocol support when generating URL helpers, for routes with specific protocol specified.
* Adds support for "protocol relative" base URL value in `:url_links` config.

### Changes:
* `generate_url_link` no longer validates that the `:url_links` config value contains the protocol.

This PR fixes #127.
If `url_links = "ftp://example.com" `(for example), then I will get "http://ftp://example.com/some_url" by this code.