[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 see your point, but I’d still prefer it to be changed to be more semantic here. The other occurrence (`add_pod_group`) can be more easily understood by the reader of the code because it’s specifically about adding files to ‘development’ group in the project.

How about: `reflect_file_system_structure_for_development` ? Long, but very clear. This same name can then also be used in the file references class instead of `group_if_development`, which will make it easier to browse the code.