Lots of changes! But it's actually solving problems consistently!

- added a default indent to the display methods
- moved to using underscore's random() function
- made the parameter names in the makeRandomTree function a lot more descriptive
- implemented the makeRandomTree function by the book, so it chooses randomly at each step
- refactored the buildHiddenSet function to use underscore's map method to directly return the generated list
- made sure to use the clone in the score function
- if we're done, don't do anything!
- not currently using the selectIndex function. in stead, just randomly choosing.
- not currently using the pruning function