some minor modifications
updating the imports and updating to the new model class.
first go at implement @iguananaut's new modeling API into specwcs.
added the new astropy API to specwcs lookup model
updates specwcs to @embray's new model API.
some minor bugfixes
Merge branch 'master' into specwcs_update

Conflicts:
	specutils/wcs/specwcs.py
added the suggestion of ditching .dispersion and using wavelength, frequency and energy.
implemented simple fits reader.
added unit support and checking. the reader can now read the simplest of fits_files (crval1, cdelt1, ..).
some smaller fixes for the wcs reading/writing
Merge branch 'master' into fits_readers
Merge pull request #36 from wkerzendorf/specwcs_update

specwcs update to new modeling API
Merge pull request #37 from wkerzendorf/dispersion_change

Dispersion change
added the fits_capable_wcs list (checking for `.from/to_fits_header`)
Merge branch 'master' into fits_readers
Merge pull request #38 from wkerzendorf/fits_readers

Fits readers
added a simple legendre 1d wcs
minor error with unit
silly "return" mistake