Jim Cramer, the host of “Mad Money” on CNBC, says his sources have told him that Amazon is prepared to go as low as needed to keep Walmart from encroaching on the e-tail giant’s market share, regardless of the category.  “They hate, hate, hate Walmart. Walmart is the biggest grocery chain in the country,” said Mr. Cramer. “Amazon will stop at nothing to take away the share that Walmart has gained.”