TooManyBuckets test failures against AWS

AWS only allows 100 buckets by default and s3-tests has over 300 tests. Since s3-tests only nukes its buckets after the end of all tests, this results in many spurious failures:

```
Ran 305 tests in 250.788s

FAILED (SKIP=4, errors=158, failures=10)
```

Any suggestions on the least invasive way to address this? nosetests allows defining a single teardown for all test methods in a class but not for all functions in a source file:

http://nose.readthedocs.org/en/latest/writing_tests.html