refactored data.py to facilitate using Learner.predict() programmatically
added keywords for some kwargs
added an optional ids_to_floats config option (defaulting to False).  added unit tests
added note about ids_to_floats in documentation
changed assert to eq_ in some tests
Merge pull request #45 from EducationalTestingService/feature/convert_data_for_predict

Don't convert IDs to float by default anymore, and added ids_to_floats option to config file.
Also added a utility function for converting a list of dictionaries to an ExamplesTuple directly.