Refactored overlay.js
Do not query for treenode tags if these are not to be displayed.
Apparently the request variables are pure text, and "false" is then
a non-empty string, which is truthy in python.
Remove debugging statement.
overlay.js: fixes to documentation.
overlay.js: fixed goToNearestOpenEndNode ('R' key binding)
Fixed documentation for tree_util.edge_count_to_root