--bs arg and --bs_percentile for bootstrapping vw

bootstrapping vw to get confidence interval on predictions. Implemented as a reduction to gd.
--bs arg to specify arg numbers of bootstrapping rounds.
--bs_percentile arg (such as 0.95) to get 0.95 percentile in -p.

Example:
in training: 
./vowpalwabbit/vw -d multiclass -k -c --passes 2000 --holdout_off -f model --bs 100
for testing:
./vowpalwabbit/vw -d multiclass -k -c -p result.txt -i model --bs 100 -t --bs_percentile 0.95

Then in result.txt we get:
```
1.645815 1.543533 1.725334
2.532156 2.468851 2.575895
3.527597 3.478951 3.574625
4.556210 4.514622 4.596277
5.475436 5.451206 5.500712
6.129002 6.075596 6.164859
6.529974 6.453068 6.593857
6.790787 6.671144 6.866988
6.961917 6.856057 7.052547
7.075695 6.946770 7.191328
```
The first number is final_prediction (mean value of bootstrapped predictions), followed by the lower bound and upper bound of confidence interval of the specified percentile.