Fix issue with multiple return values

[tjbcg]

This fixes issue #132 by allowing multiple return values

[Shouqun]

breaks test case: https://github.com/Shouqun/node-dbus/blob/master/test/client-call-object.test.js#L17

[AndreasBackx]

Adding a method which returns multiple values is also not accounted for, though multiple in arguments are supported. It should be corrected here.

https://github.com/Shouqun/node-dbus/blob/7c87b54cb5a107e6b3d3c610cc1f64cd2d26c4ec/lib/service_interface.js#L235-L239

Currently the inspection for out arguments results in this:

<arg direction="out" type="undefined"/>

[AndreasBackx]

@Shouqun any chance this could get fixed/looked at? There are perhaps some more things that need a look at?