Only take the lock when the ref count hits zero
Merge pull request #12 from pcwalton/slow-drop

Only take the lock when the ref count hits zero
Fix race in ref count manipulation in slow-drop
Merge pull request #14 from pcwalton/slow-drop-fix

Fix race in ref count manipulation in slow-drop
Merge pull request #10 from kmcallister/dev

Various enhancements
Upgrade to rustc 2014-09-15
wip