added use_bootstrap flag and support for bootsrtrap samples instead of StratifiedKFolds
added use_bootstrap flag, changed db_file and data_file args to positional,
changed verbose flag to default to false
changed to barf if db file exists when models are specified
handled ValueError from init in main()
modified args to match ensemble_train
tweak doccstring
changed version number, changed test size default to 0.75 for ensemble_train.py,
doc tweaks