Bind a client to a SOAP address 

The client is currently stateless since the `Send` method receives the URL, action and envelope. Since the client is usually used to communicate with a single SOAP server, always sending the URL is redundant. 

Possible solution: create a wrapper that always uses the same URL

``` csharp
using(var client = SoapClient.Connect("<server url>"){
    client.Send("<action>", envelope);
} 
```
