Fixed signal handling

In Node-dbus 0.0.8, signals did not appear to be working - at all. It seems that a signal filter
was added only ONCE, which is unacceptable if we have multiple connections (multiple interfaces).
In this case, the filter must be attached for each connection.

So I moved dbus_connection_add_filter to GetInterface
Merge pull request #38 from Ivshti/patch-3

Fixed signal handling