switched to AB as default mag system
  * formerly we used Vega.  Now we have --AB or --vega to explicitly pick
making it work for flts
small tweak to handle astropy v0.4
  * minor change in astropy.io.ascii.read requires 'data_start'
   keyword when using 'header_start' for commented_header reads.
strip punctuation from filter names
  * for pseudo-filters like '~f153m'
better verbosity, change in option names
  * SNANA output lines now include zero points
  * --skyisempty has become --skyalgorithm (and defaults is to use sigma-clipping)
  * small tweaks to output formatting.
bug fix for negative fluxes, change in option name
  * skyisempty option has become skyalgorithm.
  * bug fix for converting negative fluxes to lower limit magnitudes (and better
    internal variable naming