Add config.measureValue callback

config.measureValue is call with the elapsed time in milliseconds for every measurement.
This is intended to be used in Chromium Performance Test.

R=tyoshino@chromium.org

Review URL: https://codereview.appspot.com/141390043

git-svn-id: http://pywebsocket.googlecode.com/svn/trunk@844 4ff78f4a-9131-11de-b045-6380ec9940d4