Improve shared group backtrace in failure output.

- Show all shared group inclusion stack frames,
  not just the inner-most one.
- Make it work for all methods of including
  shared example groups. Previously, it only
  worked for the methods that generate a nested
  group.

Fixes #1209.
Or prehaps as this isnt really a data structure, just declare the attr_readers and add the initialize method normally? Seems we're jumping through hoops.