Reinstate accidentally deleted file header.
Fix bug that cleared tree when enabling undo-tree-mode.
Tweak undo history file names to match Emacs backup file names.

Fix undo-tree-auto-save-history docstring to make it consistent with what
actually happens.
Bump version number and copyright year.
Clear undo-tree-visualizer-needs-extending-[up|down] before drawing tree.

Fixes bug that could trigger an error when redrawing the tree after toggling
timestamps.