Utility object for setTimeout
From a once over:


I would not use _ to prefix all your variables, it would make more sense to either actually use private variables or write a comment that this variables should not be touched by callers
pauseTimer lies, since you toggle. I would call it toggleTimer or startStopTimer
Your code is well commented except for //toggles the _paused variable which is kind of obvious ;)
Besides some missing semicolons, your code looks fine on JsHint.com
I think in callTimer you could have called your paramater callback, then do callback = callback || this.callback and then use callback in your closure.


All in all, good, re-usable code. 

For my own purposes I would probably make the private variables really private and move the functions out of prototype into the constructor. That would increase memory footprint, but I would never use more than a dozen of these at the same time.