1)Use threading.Lock for token_exchange_lock.
2)SetLock function, so that gsutil can keep using multiprocessing.Manager().Lock()
3)Set boto.UserAgent (if env var is present).
4)Rename GSUTIL_DEFAULT_SCOPE -> DEFAULT_SCOPE
Merge pull request #5 from ktintc/master

Fix a bug with locks: plugin is verified to work in a Docker Container