Accept Documentation Element in wsdl, allow customized attributes like XMLSchema-instance

modifications partially copied from this fork https://github.com/MauriceSchoenmakers/node-soap

Mainly enabling complex types that inherit from / extend other complex types. For example: http://www.ibm.com/developerworks/websphere/techjournal/0401_brown/brown.html

Such attributes can be defined in request this way:
{productId:{
       '@':{'xmlns:xsi':'http://www.w3.org/2001/XMLSchema-instance','xsi:type':'{namespace}SubscriptionIdS'},
       subscriptionId:12345
 }

Also fixed "AssertionError: Invalid child type" on documentation node as described in 
https://github.com/milewise/node-soap/issues/57