Several changes to handle standard FITS spectra files

1.  Added DC-FLAG handling
2.  Dealt with BZERO (maybe not quite right..)
3.  Added error file to read_fits.read_fits_spectrum1d
For now we are still working on a unified interface for uncertainty handling and I think it that a second file might be used in your applications, but not in many others. My feeling is that this would make a great addition to the documentation for now, but not necessarily in code.