Add support for optional SOAP options and non-WSDL mode

[clue]

The Client constructor now accepts an array of options. All given options will
be passed through to the underlying SoapClient. However, not all options
make sense in this async implementation and as such may not have the desired
effect. See also SoapClient
documentation for more details.

If working in WSDL mode, the $options parameter is optional. If working in
non-WSDL mode, the WSDL parameter must be set to null and the options
parameter must contain the location and uri options, where location is
the URL of the SOAP server to send the request to, and uri is the target
namespace of the SOAP service:

$client = new Client($browser, null, array(
    'location' => 'http://example.com',
    'uri' => 'http://ping.example.com',
));

Builds on top of #31
Resolves / closes #5
Refs #23