Merge pull request #45 from wkerzendorf/wcs_linear1d2polynomial

convert LinearWCS to Polynomial model
removing non-convertible wcs attributes and lookup_table caching mechanisms (thereby also removing one of @keflavich's concerns.
fixing up attributes stuff
remove unit requirement from lookuptablewcs.
remove unit requirement form polynomial wcs
updated spectrum1d to work with and without units.
deprecating valid units for WCS.
fixed a problem with overwriting the class dictionary _wcs_attributes
added `.velocity` to wcs attributes.
adding spectral_axis term but insisting on it being 0 (for now).