scorer.cc: add --link logistic1 for f(x) -> [-1, 1]

The very common case of [-1, 1] labels is not handled as expected by `--link logistic`.  It gives predictions of 0 for the -1 labels, resulting in a high average loss overall.   The solution is to add another link function called `logistic1`.

With `--link logistic` and presence of -1 labels:
```
    average    since         example     example  current  current  current
    loss       last          counter      weight    label  predict features
    ...
    0.475941   0.951882            4         4.0  -1.0000   0.1479        2
```
with `--link logistic1` we now get what we expect, (and consequently, a much smaller loss):
```
    average    since         example     example  current  current  current
    loss       last          counter      weight    label  predict features
    ...
    0.270670   0.000006            4         4.0  -1.0000  -0.9998        2
```