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.
Oh...you're right...my mistake. So the behavior is the same as it has been. Personally, I don't feel like you should have the ability to hard-code the protocol, and it shouldn't be limited to http/https like you have said. I would prefer that the route definition has the authority to override the scheme. This would also clean up the branching code quite a bit. The code could change to `uri = URI.parse(@options[:url_links])` and you check if a protocol has been specified using `if uri.scheme`. You could assume that if a protocol was passed as an argument (which would have been specified in the route definition), then to use that protocol, no matter which protocol was specified as the config value. Furthermore, I think the names of the config options should change. I think the option `url_links` should be `true/false` and a second config option of `default_host` should be added. A route definition can even specify a specific host to use, so it would be nice to support this feature too. These changes would make js-routes act more like Rails and I think it would be more intuitive. However, those changes would require a major version bump, but I think it would be worth it.