Include TeraChecksum in Hadoop mapreduce examples driver

In the mapreduce examples driver, you can run TeraGen, TeraSort, and TeraValidate. To my surprise, TeraChecksum wasn't included in the examples driver. I think it'd be nice to include. It can be used to compare the checksum of the input & output. It's worth noting that TeraValidate calculates the checksum as well.

Patch to be posted shortly. I tested against 2.7.1 branch but couldn't against master. I am including patches for both. Patch also includes TeraChecksum test case addition.