Fixing models

Based on https://github.com/astropy/specutils/pull/92. Making models compatible with the latest astropy model framework
Fixes: https://github.com/astropy/specutils/issues/93 partially. 
IRAF specific models will be fixed in https://github.com/astropy/specutils/pull/86.
@wkerzendorf @nhmc How does this look?
Using `@classmethod` here doesn't *hurt*, but generally I just make @staticmethod if the `cls` variable isn't actually used.  It's more for documentation purposes than anything.