new features and fixes for jobs/tasks/logs/files

- spawn2 ui fields should correctly save changes in many more cases.
  In particular, using only mouse actions should now work fine (eg.
  right-click-paste, using the previously treacherous up/down arrows)
- slightly more flexible rendering for task-log views on narrower
  browser windows
- minion file tree statistics upgraded a bit to use java7's file tree
  visitor. Maybe slightly more performant, but at least cleaner
- you can now view historical task logs from the spawn2 ui. There is
  a new numeric spinner field labeled "runs ago" to control this.
- stdout/stderr logs are now preserved when reverting jobs/tasks. They
  can be viewed (for now at least) from the task log panel by setting
  "runs ago" to a negative value (larger negatives -> older archived
  error logs).