Please add endpointurl option, for use with others amazonS3 providers

[alexstep]

https://github.com/alexstep/S3

https://github.com/alexstep/S3/blob/master/server/sign_request.coffee

if ops.endpointurl is ""
    # Identify post_url
    if ops.region is "us-east-1" or ops.region is "us-standard"
        post_url = "https://s3.amazonaws.com/#{ops.bucket}"
    else
        post_url = "https://s3-#{ops.region}.amazonaws.com/#{ops.bucket}"
else
    post_url = "#{ops.endpointurl}/#{ops.bucket}"