Changing IAddon to allow clients to use HoloEverywhere without needing to switch to JDK7

This is a small change that has zero impact inside HoloEverywhere and I strongly suspect it is extremely unlikely to have any adverse impact on any HE clients. And the worse case impact is that client would need to cast the output of IAddon#get to the expect type.

The upside is that HoloEverywhere would no longer enforce a dependency on JDK7 on clients.