nycflights13 0.2.0

• airlines: carrier columns are characters instead of factors (#2).
• airports: duplicate entry for BFT removed (#7).
• flights:
  • new time_hour variable combines year, month, day, and
    hour into a single variable (#11).
  • new sched_dep_time and sched_arr_time variables give scheduled
    departure and arrival times - these are more appropriate for connecting
    to weather data. hour and minute are now computed from the scheduled
    departure time, not the actual departure time.
  • missing tailnum now recorded as NA, not "" (#10).
• weather:
  • Includes weather data for all airports.
  • New time_hour variable combines year, month, day, hour into
    a single POSIXct variable.
  • Saved as ungrouped.