Added MTOM support for binary data

[maxime-morand]

Added an extra option named "attachments" to send binary attachements using XOP+MTOM.
No changes has been made to XML request generation, only to the HTTP request, which become a multipart/related request containing both the SOAP request and the attachments.

For example, following request :

const args = {
  "field1": "field 1 value",
  "field2": "another value",
  "attachment": {
      "$xml": "<inc:Include href=\"cid:file_0\" xmlns:inc=\"http://www.w3.org/2004/08/xop/include\"/>"
  }
}

var options = {
  attachments: [{
    mimetype: 'image/png',
    contentId: 'file_0',
    name: 'nodejs.png',
    body: fs.createReadStream(__dirname + '/static/nodejs.png')
  }]
};

client.myFunction(args, callback, options);

will generate :

Content-Type: multipart/related; type="application/xop+xml"; start="0bfcf82f-f55e-4931-b142-05429eff684c"; start-info="text/xml"; boundary=' + uuid()

--66ce22ce-58b1-44cb-9006-accdc7ebf8e7
Content-Type: application/xop+xml; charset=UTF-8; type="text/xml"
Content-ID: <0bfcf82f-f55e-4931-b142-05429eff684c>

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" >
  <soap:Body>
  <Request xmlns="http://www.example.com/v1">
    <field1>field 1 value</field1>
    <field2>another value</field2>
    <attachment>
      <inc:Include href="cid:file_0" xmlns:inc="http://www.w3.org/2004/08/xop/include"/>
    </attachment>
  </Request>
  </soap:Body>
</soap:Envelope>

--66ce22ce-58b1-44cb-9006-accdc7ebf8e7
Content-Type: image/png
Content-Transfer-Encoding: binary
Content-ID: <file_0>
Content-Disposition: attachment; filename="nodejs.png"

<binary data>
--66ce22ce-58b1-44cb-9006-accdc7ebf8e7--

[coveralls]

Coverage increased (+0.03%) to 93.273% when pulling f882494 on maxime-morand:master into 1f4af8a on vpulim:master.

[jsdevel]

i'll merge this once the build is fixed