McDonald's stock hasn't made for a great investment this year, but could it still be a good buy for the long-term-minded investor?Shares of fast food giant McDonald's (MCD 1.06%) have been falling as the company deals with an outbreak of E. coli at its restaurants. At least 90 people have become ill due to the outbreak -- including one death -- apparently due to contaminated onions served on its burgers. As McDonald's moves to address the issue, investors may not be easily convinced. And when you consider t ...