Plug Power is a lot better at selling stock than selling fuel cells.Plug Power (PLUG -13.35%) stock tumbled more than 17% in morning trading Friday after the fuel cell and hydrogen fuel production company announced the surprise sale of more than 78.7 million shares of stock in an effort to raise cash to keep itself in business. As of 2:20 p.m. ET, shares are still down 13.7%.What Plug Power is selling -- and whyPlug Power closed at $2.92 per share last night, but the company said it will be selling its stoc ...