Add timestamps to stage start/finish messages.
Add timestamps to stage start/finish messages.

BUG=chromium-os:13641
TEST=Unittests

Change-Id: I08d19ccf114fdac251cbc29d188cbf008a4e93f8
Reviewed-on: http://gerrit.chromium.org/gerrit/1930
Reviewed-by: Scott Zawalski <d0bc454f32175e2361d26fbdb5930ff6be11acc5@chromium.org>
Reviewed-by: Don Garrett <546ec366615570e68497e3c5df9092106a9808f4@chromium.org>
Tested-by: Don Garrett <546ec366615570e68497e3c5df9092106a9808f4@chromium.org>

coreos/chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,chadversary/chromiumos.chromite,chadversary/chromiumos.chromite,zhang0137/chromite