[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


Basically, the enumeration + potentially creating of multiple groups + only capturing 1 (the last one) is confusing me as to what this code does and could probably use some simplification.