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'.
Changed 'Scheme' to sum type
Merge pull request #4 from 2piix/master

Add 'gScheme' to 'GravatarOptions'