Manage endpoints

When your bucket isn't us-standard, you can't upload/download files because of a wrong endpoint (you specify "s3.amazonaws.com" in your code). In fact, you get this error message :

*The bucket you are attempting to access must be addressed using the specified endpoint. Please send all future requests to this endpoint.*

To resolve this issue, you just have to let the user specify the appropriate endpoint.
I have changed you code by adding a "-e" option which let the user specify the endpoint he wants (if not specified, use *s3.amazonaws.com*).