Recursive PublishedContentExtensions IsDocumentType

Optional recursive parameter defaulted to false for backwards compatibility and no performance hit.
Settings recursive = true fetches content type for content and checks ancestor content types for the same alias.
Split the recursive IsDocumentType into an overload for "real" backwards compatibility.

Changed string comparison to use InvariantEquals. (guessing it doesn't _really_ break backw. compat?)
Added XML docs.
Merge pull request #142 from lars-erik/6.2.0-recursive_isofdocumenttype

Recursive PublishedContentExtensions IsDocumentType