By Martin Baccardax Amazon’s surprise 20-for-1 stock split, the first in two decades, could help the world’s largest retailer tackle one of its biggest challenges. Amazon’s (AMZN) – Get Amazon.com, Inc. Report first stock split in more than 20 years has shares in the world’s biggest online retailer trading sharply higher Thursday, and may also pave the way towards its inclusion in the Dow Jones Industrial Average. The 20-for-1 split, which will begin trading in early June, will effectively reduce the cost of buying one of the most expensive stocks — at least in nominal terms — from just unde…