Remove _REGRESSION_MODELS from generate_predictions

- Replace with RegressorMixin as with everywhere else.
Updated generate_predictions.py

- Replaced _REGRESSION_MODELS with RegressorMixin.
- Modified Predictor.predict to infer if feature hashing was used and pass that to Learner.predict.
- Added logger.
- Modified file reading code to use the new Reader API.
- Added error check to skip files with unsupported extensions.
Created new tests and reorganized old ones

- Added new test_utilities.py file.
- Moved the compute_eval_from_predictions test from test_output.py to this new file.
Fixed minor bugs in generate_predictions.py
Added console mode tests for generate_predictions