[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.each_filename do|name|
          next if name == "."
          group = group[name] || group.new_group(name, name)
        end
```
I'm iterating over the relative path and creating a new sub-group for each path-component if there is no one exists.
Let say I have to add a file reference to teh path "A/B/C/D/file" to a group with real_path of "A/B".

```
relative_path = Pathname.new(absolute_path).relative_path_from(group.real_path).dirname 
# relative_path now is: "C/D"

relative_path.each_filename do|name|
  next if name == "."
  group = group[name] || group.new_group(name, name)
end

# In first iteration name is "C", group is refering to "A/B"
#   if the group has a subgroup with name "C" I'm just switching to it, otherwise I'm creating it and switching to it
# I second iteration name is "D", group is refering to "A/B/C"
#   if the group has a subgroup with name "D" I'm just switching to it, otherwise I'm creating it and switching to it
#
# After the loop the group is referencing to "A/B/C/D" and I did create (if they weren't there) the subgroups "C" and "D" under the original group referencing to "A/B"
```