Define ID_ALLOCATOR to something harmless if not defined, needed for Ruby 2.0.0
Update GET_THREAD for Ruby 2.0.0

Based upon https://github.com/windwiny/debugger/commit/89dffcaa1b69fe97082ea6a664c7a66f61f5f9fb
Merge pull request #56 from chneukirchen/fix-on-ruby-2.0.0

Fix for Ruby 2.0.0