[boost] [timer] update

Hi guys, for the reasons noted in my previous message about boost::timer, I've updated it to report a custom exception in case it cannot (re)start the timer. If there are no objections I'll change the docs to reflect this. The modification is on the trunk only, of course. http://boost.cvs.sourceforge.net/boost/boost/boost/timer.hpp?r1=1.7&r2=1.8 --Gennaro.