Require 'spec_helper' in acceptables spec
Additional thread safety fixes

The original fix was not complete, it only synchronized around closed?,
not around close itself. Pretty dumb, but this commit fixes it properly.

Ran it in a loop with Rubinius for quite a while and didn't see any
crashes anymore.
Merge pull request #11 from dbussink/additional_thread_safety

Additional thread safety fixes
Bump version to 0.4.3
Merge pull request #12 from loganb/master

Fix for broken monitor spec
Merge pull request #13 from loganb/master

Fix broken specs on OS X 10.8
Spec for selecting closed IO objects