Add optional time stamping and elapsed time reporting to Trace().
Add optional time stamping and elapsed time reporting to Trace().

timj/scons,timj/scons,andrewyoung1991/scons,andrewyoung1991/scons,andrewyoung1991/scons,andrewyoung1991/scons,andrewyoung1991/scons,timj/scons,andrewyoung1991/scons,timj/scons,timj/scons,timj/scons,andrewyoung1991/scons,timj/scons,timj/scons,timj/scons,andrewyoung1991/scons,andrewyoung1991/scons