build response: add methods for retrieving tar metadata
build response: add methods for retrieving tar metadata

dock: https://github.com/DBuildService/dock/pull/127

Fixes #111

jpopelka/osbs-client,pbabinca/osbs,bfontecc007/osbs-client,pombredanne/osbs-client,DBuildService/osbs-client,projectatomic/osbs-client,pbabinca/osbs,twaugh/osbs-client,vrutkovs/osbs-client,DBuildService/osbs-client,bfontecc007/osbs-client,jpopelka/osbs-client,pombredanne/osbs-client,projectatomic/osbs-client,vrutkovs/osbs-client,twaugh/osbs-client