one more import variation  and   async schema uri resolving for server creation

These changes enable schema imports like the following  (schema has no attributes, and the imports are single schemas):

```
<wsdl:definitions  ... >
	<wsdl:documentation>...</wsdl:documentation>
	<wsdl:types>
		<xsd:schema>
			<xsd:import namespace="http://valueobjects.xx.com" schemaLocation="valueobjects.xsd"/>
			<xsd:import namespace="http://exceptions.xx.com" schemaLocation="exceptions.xsd"/>
			<xsd:import namespace="http://enums.xx.com" schemaLocation="enums.xsd"/>
		</xsd:schema>
	</wsdl:types>...
```

furthermore I changed the options for creating a server to allow a 'uri' for the wsdl with a callback (as for the client) and not just the xml. 
The reason was that the relative schema locations above, could not be resolved. But they were resolved when loading via the uri in the createClient case)

and changed WSDL.prototype.objectToXML, so a user can define explicit attributes to be used in the xml:
```
{productId:{
       '@':{'xmlns:xsi':'http://www.w3.org/2001/XMLSchema-instance','xsi:type':'valueobjects:SubscriptionIdS'},
       subscriptionId:12345
 }
```