Scheme shouldn't be prepended to the home URL if it's already present.

Currently, eve-docs unconditionally prepends the `PREFERRED_URL_SCHEME` the `home_link()` URL reported by the app. Eve's current development branch supports a new [`URL_PROTOCOL`](http://python-eve.org/config.html#global-configuration) setting, though. If it is set (because the user desires absolute URLs in HATEOAS output), then the scheme will already be present in the URL. When eve-docs prepends another scheme, the resulting URL is invalid. eve-docs should instead detect the presence of the scheme and refrain from making the change.

I'm submitting this change now because it's easy to implement in a backward-compatible way.

P.S. Thanks for your work on eve-docs. I'd love for you to release it on PyPI.