Meter: the start time is not initialized correctly

Currently the start time of meter is initialized once the mark function is called first time. But I think it's too late .

Considering the following code:
```js
var timer = stats.timer('query').start();
var end = Date.now() + 500;
while (Date.now() < end) ;
timer.end();
stats.end();
```
The expected mean rate should be around 2.

I have moved it to the constructor of Meter and modify the tests accordingly.