Remove thread locals

This PR replaces Thread local storage with instance variables so as to encapsulate state within the object.

Running on Rubinius 2.0.0.rc1 (1.9.3 be5c0cb8) rake test produces:
```
Finished in 1.47699s
272 tests, 1107 assertions, 0 failures, 0 errors, 0 skips
```