Accommodate differences in Thread class under MRI 1.9.3, MRI 2.0.0 and jruby.

On a killed thread, Thread#alive? behaves differently under MRI 2.0.0 than under MRI 1.9.3, and jruby and Thread#status behaves differently under jruby than under MRI 1.9.3, and MRI 2.0.0.
t=Thread.new{}.kill; puts "#{t.alive?}--#{t.status}"
  On MRI 1.9.3 => true--aborting
  On MRI 2.0.0 => true--run
  On jruby        => false--aborting
Merge pull request #486 from cablegram/fix-thread-status

Accommodate differences in Thread class under MRI 1.9.3, MRI 2.0.0 and jruby