added basic SpectralUnit functionality
added basic SpectralUnit functionality

pllim/astropy,bsipocz/astropy,AustereCuriosity/astropy,tbabej/astropy,pllim/astropy,mhvk/astropy,funbaker/astropy,StuartLittlefair/astropy,stargaser/astropy,StuartLittlefair/astropy,MSeifert04/astropy,StuartLittlefair/astropy,saimn/astropy,astropy/astropy,astropy/astropy,tbabej/astropy,AustereCuriosity/astropy,kelle/astropy,kelle/astropy,mhvk/astropy,joergdietrich/astropy,tbabej/astropy,dhomeier/astropy,dhomeier/astropy,MSeifert04/astropy,DougBurke/astropy,lpsinger/astropy,AustereCuriosity/astropy,lpsinger/astropy,mhvk/astropy,lpsinger/astropy,astropy/astropy,mhvk/astropy,kelle/astropy,DougBurke/astropy,tbabej/astropy,stargaser/astropy,MSeifert04/astropy,saimn/astropy,saimn/astropy,larrybradley/astropy,joergdietrich/astropy,dhomeier/astropy,aleksandr-bakanov/astropy,funbaker/astropy,pllim/astropy,stargaser/astropy,tbabej/astropy,DougBurke/astropy,funbaker/astropy,funbaker/astropy,pllim/astropy,larrybradley/astropy,larrybradley/astropy,kelle/astropy,joergdietrich/astropy,kelle/astropy,lpsinger/astropy,astropy/astropy,MSeifert04/astropy,StuartLittlefair/astropy,dhomeier/astropy,AustereCuriosity/astropy,bsipocz/astropy,larrybradley/astropy,saimn/astropy,aleksandr-bakanov/astropy,larrybradley/astropy,aleksandr-bakanov/astropy,bsipocz/astropy,pllim/astropy,bsipocz/astropy,DougBurke/astropy,AustereCuriosity/astropy,lpsinger/astropy,saimn/astropy,stargaser/astropy,dhomeier/astropy,mhvk/astropy,StuartLittlefair/astropy,astropy/astropy,aleksandr-bakanov/astropy,joergdietrich/astropy,joergdietrich/astropy