[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


relative_path is a relative path, in certain case it can be ".":
```
irb(main):003:0> Pathname.pwd.relative_path_from(Pathname.pwd).each_filename {|x| p x}
"."
```
In our case it happens when the file is in the directory bound to the group.