[#159] Pass custom options and headers for WSDL requests

Greetings,
This allows custom headers and options to be passed as part of the `createClient` call.  For example:

```coffeescript
soap.createClient wsdl_url, { 'endpoint': endpoint, wsdl_options: { strictSSL: false } }, cb
```

This passes `wsdl_options` and `wsdl_headers` down to the `open_wsdl` call, which removes them before passing them on, to be safe.

--  Morgan