Allow override of Amazon S3 API endpoint

I was having problems uploading to my bucket because the form url was incorrect. Added variable url to the configuration file in case someone needs to override it just like I did.

My new configuration file is as follows:

    S3DirectUpload.config do |c|
      c.access_key_id = ""       # your access key id
      c.secret_access_key = ""   # your secret access key
      c.bucket = "" # your bucket name
      c.region = ""              # region prefix of your bucket url (optional), eg. "s3-eu-west-1"
      c.url = "https://#{c.bucket}.s3.amazonaws.com/"
    end