Work around boto/boto#1898 (AttributeError: _strptime)

Boto (used by s3funnel) can trigger Python issue 7980, where an AttirbuteError can be raised when using multiple S3Connections concurrently on different threads. The workaround is to call datetime.strptime once on the main thread during initialization. This commit implements the work around until boto/boto#1898 is fixed.