On May 2nd, Walmart sent to me to my local store with a challenge. How many groceries could I get for my family of four with $100? Before I get to the big reveal, let me tell you that "frugal" has been an f-word we've been using around here for awhile now. With a recent kitchen renovation, a new roof due, a sick dog and a car on the fritz, penny pinching has become my latest obsession. I have regularly used Walmart to stretch my dollar but I have to admit that after last week's challenge, …