** SunEdison Inc agreed to buy Vivint Solar Inc , the second-biggest U.S. solar panel installer, in a deal valued at about $2.2 billion, speeding up its expansion in the booming residential solar market. ** Italian holding company Exor has again sweetened its offer for U.S.-listed reinsurer PartnerRe to include a special dividend of $3 per share for common shareholders if it is chosen over rival suitor Axis Capital Holdings.