Sampler with Automatic Cross Validation support

Usage:
```
  # Feature library gives us lots of features so we need proper
  # regularization. Use "--reg_param X" (or -b X) multiple times and 
  # system will choose proper parameter with Cross Validation.
  sampler.sampler_args: "-l 300 -s 1 -i 500 --alpha 0.1 --diminish 0.99 --reg_param 0.1 --reg_param 1 --reg_param 10"
```