changes Time to be able to output negative values correctly with a minus sign
at the beginning.

it still can't accept negative times from fromUnitString, but there's no need
for it to be able to.

there's really no need for it to be able to display negative times either, as
they shouldn't happen, but at least if there's some bug where negative times
come up the display won't go completely messed up.

adds basic tests for negative times.
Clock - corrects check for whether to subtract thinking time - last
move index needs to be at least the first timed move index - 1