Autopadding option

First, this patch fixed the travis tests error in #5(https://github.com/auth0/node-xml-encryption/pull/5)

If developer sets options.autopadding = false, padding will return to developer, he/she can handle it as his/her will.
Otherwise(options.autopadding = true or not set), padding will be cut before return to developer or throw padding length error if padding length is not in the range 1 to 16