Reintroduce time_fraction_threshhold

[gak]

This feature would filter out nodes that have taken less time than this argument, as a fraction of the total time. e.g. a value of 0.1 for a trace that took 10 seconds would discount nodes that took less than a second.

This was taken out in the last minute because it wasn't reimplemented in the 1.0.0 major refactor. If there's enough need for it, it should be reimplemented.