add actor.wait return value

[ahappyforest]

while true do
    local ret = actor.wait({
        recv = function(msg)
            return "hello world"
        end
    })
end

sometimes I think It's very useful ; -)

[coveralls]

Coverage remained the same when pulling 085bbbe4b43a8abb3360055422cf5b16cbac06f7 on van9ogh:patch-3 into 0ac606c6acca8b364462c8040f58caeda04b166d on xfguo:master.