copy unit registry instead of completly rebuilding it.
copy unit registry instead of completly rebuilding it.

also included: fixed some PEP8 issues

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