added to_selectable() onto ColumnClause to simplify _get_col_by_original
added to_selectable() onto ColumnClause to simplify _get_col_by_original

hsum/sqlalchemy,sandan/sqlalchemy,Cito/sqlalchemy,wujuguang/sqlalchemy,276361270/sqlalchemy,WinterNis/sqlalchemy,bdupharm/sqlalchemy,monetate/sqlalchemy,davidfraser/sqlalchemy,ThiefMaster/sqlalchemy,Cito/sqlalchemy,bootandy/sqlalchemy,zzzeek/sqlalchemy,itkovian/sqlalchemy,hsum/sqlalchemy,Akrog/sqlalchemy,inspirehep/sqlalchemy,halfcrazy/sqlalchemy,ThiefMaster/sqlalchemy,wfxiang08/sqlalchemy,davidjb/sqlalchemy,wfxiang08/sqlalchemy,elelianghh/sqlalchemy,monetate/sqlalchemy,robin900/sqlalchemy,brianv0/sqlalchemy,robin900/sqlalchemy,EvaSDK/sqlalchemy,pdufour/sqlalchemy,inspirehep/sqlalchemy,graingert/sqlalchemy,halfcrazy/sqlalchemy,davidjb/sqlalchemy,olemis/sqlalchemy,epa/sqlalchemy,Akrog/sqlalchemy,wujuguang/sqlalchemy,bdupharm/sqlalchemy,bootandy/sqlalchemy,alex/sqlalchemy,alex/sqlalchemy,graingert/sqlalchemy,dstufft/sqlalchemy,j5int/sqlalchemy,davidfraser/sqlalchemy,j5int/sqlalchemy,sqlalchemy/sqlalchemy,276361270/sqlalchemy,EvaSDK/sqlalchemy,alex/sqlalchemy,sandan/sqlalchemy,epa/sqlalchemy,WinterNis/sqlalchemy,dstufft/sqlalchemy,Cito/sqlalchemy,pdufour/sqlalchemy,olemis/sqlalchemy,elelianghh/sqlalchemy,brianv0/sqlalchemy,itkovian/sqlalchemy