output messages to the specified stream instead of STDOUT

This fixes a strange behavior of the `show` methods of `MCMCChain` and `MCMCLikelihoodModel`.
The behavior will be reproducible as follows:

```julia
chain = run(mymodel, RWM(0.1), SerialMC(steps=1000, burnin=100))
@show chain
```

The output is:
```
3 parameters, 900 samples (per parameter), 0.4 sec.
chain =>
```

You can see that the description message comes before the arrow (`=>`).

The expected behavior would be:
```
chain => 3 parameters, 900 samples (per parameter), 0.4 sec.
```