Change to second (not minute) precision

[tevpg]

Needs:

• a .full property (new & old) which is full timestamp
• 'self' remains HH:MM
• change .num to .num_minutes for clarity (& consistency with new .num_seconds property)
• database structure
• JSON read/write
• frequency graphs do not work with HMS for some reason
• clarify what acceptable inputs to VTime() are:
  • 'now'
  • maybe 'now+/-[minutes]'
  • consider 'earlier' and 'later' keywords, which would be +/- 15 minutes
  • HHMM, HMM, HH:MM, H:MM (for input from humans)
  • HH:MM:SS or H:MM:SS, probably as inputs from database and JSON