BUG-413812

Mark RateTracker as Serializable to allow it to be stored in a Session that is
eventually persisted in a distributed memory platform.

Signed-off-by: David Harrigan <dharrigan@gmail.com>
Merge branch 'jetty-7' into release-7
set for release
[maven-release-plugin] prepare release jetty-7.6.12.v20130726
[maven-release-plugin] prepare for next development iteration