Fix Thread leaking in ThreadLocalVariables

- introduce #bind method to avoid value leaks
- warn on using just #value=
Good point about item 3. Even though we won't be using `Ref` in our JRuby gem, it will still be installed if we list the dependency in the gemspec. Compilation would fail on install even if we never load `Ref` at runtime. But now that I think it through, we can easily solve this. Since it isn't being used under JRuby I can added a guard in our gemspec to *not* require `Ref` when we create our pre-compiled JRuby build. Then `Ref` will never be installed.

I'll make the update on this branch later this evening.