Timers need some work.

I think something much like custom stats, custom counters is needed for timers.
(Personally, i miss the `std::clock`-based timer.)
The `UseManualTime()`/`UseRealTime()` should also be just one `UseTimer(name)`.

Also in v2, once json is here, it would be great to be able to read those values,
and generate custom stats based on them.

I'm also wondering if there are any sane ideas for the interface for these timers.
Currently, there is only a `SetIterationTime()`, which needs to be called every iteration.