Better handle multiple instances of Timers

[cafreeman]

Right now, gjallarhorn is completely naive about the fact that the Performance API will gladly create multiple measures for the same set of marks. It just grabs the very first one every time, which means that if you create multiple timers with the same marks and don't call clear(), you'll get trolled by the fact that you'll only ever see one instance of that Timer's measures.

I'd like to handle this by also creating some kind of hash that we track internally to differentiate the marks, while keeping the labels clean.