.obj instead of objectMode

`though2` exposes a `.obj(transform, [flush])` method that converts the stream to objectMode.
The difference between this and using `though({objectMode:true}, ...)` is that it also sets `hwm` to 16 instead of 16k (a bug that is fixed in node 0.12 as well). Without the `hwm` fix backpressure will (almost) never be used and the stream could end up using a lot of ram.