Fix inconsistent next and previous pointers

[donkawechico]

See #20.

Overview

• Changes History.traverse() to only set previous and next pointers if move doesn't already have pointers set.
• Adds two unit tests that recursively checks ALL variation lines in a PGN and ensures self-consistency

This fixes (what I believe to be) a bug in traverse where a move in the main line will get its next property set to the first move of an alternate line rather than the next move in its own line.

In general, it seems the correct behavior ought to be that every move in move.variation should satisfy

move.variation[0] <--> move.variation[1] <--> move.variation[2] <--> ... <--> move.variation[n]

If one of those moves has variations branching off it, that shouldn't matter to that move's previous and next pointers.