Issue #50 Add a new job history key called "diagnostics" in hadoop2

The new field will be added as per patch in MAPREDUCE-5648 and YARN-1551.
These jiras are to add a reason (message / diagnostics) as to why a job got killed.
(current patch there does not preserve the reason in the history file, but that patch-update will be forthcoming shortly).
Adding an additional key is backwards compatible, meaning that history file that do not have it can still be processed w/o breakage.