Add specs for ThreadGruop#enclose and ThreadGroup#enclosed?

Based on MRI docs for ThreadGroup, passes with Ruby 1.9.3
Add spec for ThreadGroup::Default constant

Based on MRI docs and passing in MIR 1.9.3
Add ThreadGroup::Default constant according to spec
Implement ThreadGroup#enclosed? as per spec
Implement ThreadGroup#enclose as per spec
Merge pull request #2402 from rmg/thread_group

Add ThreadGroup::Default, ThreadGroup#enclose, and ThreadGroup#enclosed?