tools: improve user experience by choosing best certificate
automatically and print final response in a pretty way

ndns-update
        1) pick the best certificate to sign the Update automatically
        2) support multiple token for content option

ndns-dig
        1) enhance output option

find a bug that validator throws exception when getting a NDNS-NACK
refs: http://redmine.named-data.net/issues/2212

Change-Id: Ib06981c224ccad2a122a4505c84a60977184a55b