Make all nodes case classes or have companion object constructor

[tomerk]

Discussion with @etrain suggests that we should be very consistent about how nodes are constructed.

Because:

• @etrain doesn't like new
• including empty parens in some cases (case classes w/ default parameters) and not others (objects) is inconsistent

The plan is to have all nodes either be case classes, or have a companion object constructor

[etrain]

To be fair - I have no problem with new - it's just a dislike for boilerplate.

Guidelines about where and when to use objects vs. case classes could also go in a style guide. #113