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?
The `self.__class__` isn't necessary :)