Patch for issue #150

Not being able to add xmlns to soap headers.
Merge pull request #350 from ash14/master

Patch for issue #150 - Unable to add namespace to SOAP headers.
Added some documentation on options object when calling createClient.
Merge pull request #351 from goloroden/master

Added some documentation on options object when calling createClient.
Introduced `$lookupType` and `$lookupTypes` on a `message` `part` element which has declared an `element` instead of a `type`.

Within `Client.prototype._invoke()` the `input.$lookupType`  property will be used if `input.$type` is not available, providing a first hook for further parameter look ups to move in the right direction and resolve the desired property with the already existing/implemented code.