Add more options to the FITS to PNG script. Make it output JPEG as well.
Add time formatting and improve FITS headers.

  * Add `format` method to the WFTime structure.
  * Use formatted UTC time in the names of the output files.
  * Add overloaded writeHeader method to WaterfallBackend class
    to simplify writing FITS headers and cleanup the header
    writing code a little.
  * Add DATE and DATE-OBS headers to the FITS files.
  * Change the data type of the time axis value in FITS files from float to integer.