Add PBXGroup as acceptable PBXFileReference value

Xcode can create fileRef references to PBXGroup in the PBXBuildFile
section.  This is known to happen when copying a directory from a
project template during new project creation.
Add unit test for PBXGroup

Verify PBXGroup is an acceptable value for PBXFileReference.
Merge pull request #49 from frisocki/master

Allow PBXGroup as PBXFileReference Value