add checksum verification on resume and non-resume downloads
add checksum verification on resume and non-resume downloads



GoogleCloudPlatform/gsutil,chriskuehl/gsutil-debian,dtjackson/gsutil,BrandonY/gsutil,dimfeld/gsutil,GoogleCloudPlatform/gsutil,craigcitro/gsutil,ttiurani/gsutil,fishjord/gsutil,mattdr/gsutil