Added method points_to_csv that saves known data points to csv file

[ternaus]

Added method that saves known datapoints as a csv file.

ex:

       target        alpha      colsample_bytree        gamma
        -1166.19102       7.0034                0.6849       8.3673
        -1142.71370       6.6186                0.7314       3.5455
        -1138.68293       6.0798                0.9540       2.3281
        -1146.65974       2.4566                0.9290       0.3456
        -1160.32854       1.9821                0.5298       8.7863

Format is consistent with initialize_df method and as a result allows to address #28

[fmfn]

Thanks for this PR.

I think the pandas dependency could be avoided here, though, and instead use python's CSV module.