Filter folders in "man"

Roxygen will try to access folders as if they were ".Rd" files in the "man" directory (at least in Windows).

See the following thread for more details:
https://groups.google.com/forum/#!topic/rdevtools/QIsmPk_YL_U
Filter folders in "man" (this time is right)

Add a negation to the line I previously wrote...
Merge pull request #248 from eibanez/patch-1

Filter folders in "man"