Avoid recalculating thread.total_time

`RubyProf::GraphHtmlPrinter` can be unbearably slow. After a bit of digging around, I found that `thread.total_time` was being re-calculated every time an html link was created. Since `#total_time` is pretty slow, it pays off big time to re-use it once it's been calculated once.

This PR should fix #117