added a Spectrum1D polynomial model
deprecated Spectrum1DLinearWCS
added Spectrum1D Polynomial to the fits_reader. Also added shape to the FITS keyword header reader.
skipped failing tests - both readers and testers will have to be reimplemented using the specwcs classes.
Merge pull request #45 from wkerzendorf/wcs_linear1d2polynomial

convert LinearWCS to Polynomial model