avoid sub-classing ThreadLocal since it will leak in envs such as (warbled) .war apps

due JRuby classes being loaded from the (WEB-INF/lib) application loader ...
prevents such applications from correctly releasing class memory (e.g. on re-deploys)
Merge pull request #11 from kares/thread-local-leak

avoid sub-classing ThreadLocal since it will leak in envs such as .wars