Support for regression problems

• TPOT now supports regression problems! We have created two separate TPOTClassifier and TPOTRegressor classes to support classification and regression problems, respectively. The command-line interface also supports this feature through the -mode parameter.
• TPOT now allows you to specify a time limit for the optimization process with the max_time_mins parameter, so you don't need to guess how long TPOT will take any more to recommend a pipeline to you.
• Added a new operator that performs feature selection using ExtraTrees feature importance scores.
• XGBoost has been added as an optional dependency to TPOT. If you have XGBoost installed, TPOT will automatically detect your installation and use the XGBoostClassifier and XGBoostRegressor in its pipelines.
• TPOT now offers a verbosity level of 3 ("science mode"), which outputs the entire Pareto front instead of only the current best score. This feature may be useful for users looking to make a trade-off between pipeline complexity and score.