pbxFileReference path fix for Windows

The _pbxFileReferenceObj_ function fails when run on a Windows machine. On Windows the file.path contains backslashes instead of slashes (if the file is in a subdirectory), thus creating an invalid entry into the _.pbxproj_ file.

You can see this incorrect behaviour for example in the .pbxproj file when adding plugins to a Cordova project on a Windows machine.

These suggested change replaces all backslashes with slashes to normalize the file.path..