[DON'T MERGE] Add support for nested groups.

[erickoledadevrel]

Addresses feedback received from user:

The GROUP_EMAIL only works if the group has users as its members. It doesn't work for a main group that has other Google Groups as its members. Would it be possible to add this functionality that it would work for a group of groups?

[erickoledadevrel]

I've discovered a bug with Group.getGroups() that is making this problematic. Don't merge yet.