WIP for walking ES6 nodes (accidental commit)
update Esprima

includes fixes for:
- https://code.google.com/p/esprima/issues/detail?id=526
- https://code.google.com/p/esprima/issues/detail?id=527
log warnings by default
walk children of ES6 nodes (#555)

For now, the node visitor discards these and logs a warning.