Engagement tracking

[jbothma]

Some ideas to explore the level of engagement

• Change the salary event to be the last value in the salary box before some time delay
  -- e.g. it's the same salary for keyup events < 2s apart - keyup events > 2s apart regard a new salary
• record the time since page load to the first engagement
• add an event for the salary box getting focus
• add an event for the bottom of a tax clock with salary displayed scrolling into view