version: switch to year.month numbers

version: switch to year.month numbers

As discussed on list, switch numbering scheme to be based on year/month.
Release 2.3 then becomes 16.04.

    Ref: http://dpdk.org/ml/archives/dev/2015-December/030336.html

Also, added zero padding to the month so that it appear as 16.04 and
not 16.4 in "make showversion" and rte_version().

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Signed-off-by: John McNamara <john.mcnamara@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>