Removing 'use_dense_features' and 'do_scale_features' options from the
config file and use a single 'feature_scaling' option that will
automatically convert features to dense if required.
Added a unit test for feature scaling
Removed 'scale_features' from experiments.py defaults.
Fixed bug in test_skll.py that made it fail with python 3.
Merge pull request #42 from EducationalTestingService/feature/remove_use_dense_features_option

Remove 'use_dense_features' option.