fixed up the dispersion attribute handlers

A fix up of the `.wavelength`, `.energy`, ... attributes. These are now saved in a dictionary called `wcs_attributes`:
the `.wavelength` entry is an attribute `{'unit':u.m}`. 

Thus we can easily add on attributes like `.velocity` by simply adding it to the dictionary. 

This PR will have to be updated slightly after https://github.com/astropy/specutils/pull/40 is merged. 

@keflavich - that allows `.velocity` now (among others), thoughts?