[installer] Group support added for Development Pods mirroring the folde...

Hopefully fixes https://github.com/CocoaPods/CocoaPods/issues/2145 by adding a group support for development pods.
It reflects the filesystem structure using subgroups under Development Pods group, thus eases the pod development in tandem with a host application.

Similar complain here: http://stackoverflow.com/questions/21168826/preserve-folder-structure-cocoa-pods and here: https://github.com/CocoaPods/CocoaPods/issues/648


I would like to see a more semantic name for this variable. E.g. something like `reflect_dir_structure_on_disk`.