Add support for setting xml attributes in JSON

I implemented support for setting xml attributes within an object passed as args to a service. I could not find any way to do this currently. xml2js does this by using an attribute identifier ('$' by default). I implemented this in the ObjectToXML method of wsdl.js and thought other people might find it useful.

Example:
passing this object:
{
  element1:  {
    $attributeName: 'true',
    childElement: 'data'
  }
  element2: 'data'
}

Generates this XML:
<element1 attributeName="true">
  <childElement>data</childElement>
<element2>data</element2>