Merge pull request #4 from ktintc/master

Fix a bug oauth2_client.OAuth2GCEClient -> OAuth2GCEClient
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