Knock another 50% off Time construction

Just give up on std::stringstream. It's too slow, and std::atoi is safe
enough since std::string guarentees null termination for us.

BEFORE:
 --> Time::Time(std::string const&)/Time::operator=(std::string const&) : 158418 / 11 (14401)
     Time::Time(std::string const&)/Time::operator=(std::string const&) : 95998 / 5 (19199)

AFTER:
 --> Time::Time(std::string const&)/Time::operator=(std::string const&) : 43468 / 5 (8693)
     Time::Time(std::string const&)/Time::operator=(std::string const&) : 75798 / 11 (6890)