Restore behaviour when not passing a --docrootdir parameter from Mono 2.8 when running on Mono 3.0+

Since Mono 3.0, running monodoc without specifying --docrootdir has caused an ArgumentNullException. This works around it.
Merge pull request #35 from directhex/master

Restore behaviour when not passing a --docrootdir parameter from Mono 2.8 when running on Mono 3.0+