Make it safe to run tests in parallel
Fix a race condition in test cleanup

->list_keys during $bucket->remove_all might not return all keys in the
bucket. New keys might appear after ->list_keys. If that happens, the loop
waiting for all keys to be gone won't ever terminate, causing tests to hang,
especially when run in parallel.
Ignore built dists
Make etag and last_modified optional

With older versions of Riak, these headers wouldn't always be returned.
WIP: Improve docs after recent updates