Add warm-up iteration config

The first n iterations are excluded from the average
where n is the number of warm-up iterations.

R=tyoshino@chromium.org

Review URL: https://codereview.appspot.com/147190043

git-svn-id: http://pywebsocket.googlecode.com/svn/trunk@853 4ff78f4a-9131-11de-b045-6380ec9940d4