Support submessages in multitest

[ethanfrey]

This is a collection of items to do (multiple PRs, can be broken into issues).
Follow up from #257 and #258

• Support reply method on contracts
• Process submessages if returned from contract execution

[ethanfrey]

Looks like this needs to be urgently fixed for the 0.15.0 upgrade:

            CosmosMsg::Wasm(msg) => {
                let (resender, res) = self.handle_wasm(sender, msg)?;
                let mut attributes = res.attributes;
                // recurse in all messages
                for resend in res.messages {
                    let subres = self.execute(resender.clone(), SubMsg::new(resend))?;
                    // ignore the data now, just like in wasmd
                    // append the events
                    attributes.extend_from_slice(&subres.attributes);
                }
                Ok(AppResponse {
                    attributes,
                    data: res.data,
                })
            }