[Reviewer: Alex] HTTP latency statistics

Alex,

Please can you review my changes to add HTTP latency statistics to sprout?  I've
*   added a StopWatch class that easily monitors latency
*   moved stack.cpp to use StopWatch rather than its own hand-rolled latency tracking
*   added StopWatch function to HSSConnection and XDMConnection, reporting these as new statistics
*   added code to the stats collectors (both C and Ruby versions) to read and display these statistics.

There's no UT (because we can't test latency stuff there), but I've tested this live.

Thanks,

Matt