D-Bus methods are missing in_signature and out_signature

[Tom Deseyn]

Example:
```
    @dbus.service.method('org.wicd.daemon.wireless')
    def GetWirelessProperty(self, networkid, prop):
```

in_signature and out_signature on the D-Bus methods are used to provide D-Bus introspection.

When no out_signature is provided dbus-python will start guessing the corresponding D-Bus type.

In case a method returns a different type depending on the provided arguments, use the D-Bus variant type as the out_signature.