[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 don’t understand the need for this, in my testing it doesn’t include `.` and `..`. Can you explain why this is needed?

```
$ irb -r pathname
irb(main):001:0> Pathname.pwd.each_filename { |x| p x }
"Users"
"eloy"
"tmp"
"WatchkitCurrency"
```