Add 'gScheme' to 'GravatarOptions'

The 'gScheme' option takes a (Maybe Scheme) value, and defaults to
'http' to preserve current defaults.

The 'Scheme' type is exported, as are 'http', 'https', and 'slashes'.
This looks good, but I have a couple of ideas.

- I don't think we need the `Maybe`. If anything, the case that would be `Nothing` is really `Just slashes`, and I'd call it "none".
- Technically, only "http", and "https" is the scheme. I'm not sure what to call it with the "://" appended.
- I liked your sum type idea better: `data Scheme = Http | Https | None`. At that point, maybe give it a `Show` instance that gives "http://", "https://", and "//". I'm on the fence if `| Custom String` is useful to add now, probably not.