assign background to row of .table-striped

if background is not set on row, striping doesn't work properly in responsive tables where row and cells are set to  display: block.

sorry, no bug report - jumped straight to fixing

bin
http://jsbin.com/cuyov/1/edit?html,css,output
I guess td and th backgrounds can be removed
not sure why the original decision was to style the cells, not the rows

in fact the rows aren't styled anywhere.