RFR: RecordWork instrument type for timing and counting generic functions

This new type in the instruments library lets you count and time a function call in a generic fashion.

You can use this a couple of different ways:

More explicity:
```javascript
function doSomeExpensiveOp(err, callback) {
  var timer = new RecordWork('do.some.expensive.op', callback);
  // You can explicitly get a mutated callback
  callback = timer.getCallback();
  timer.startWork();

  // timer will automatically stop when the callback is invoked.
  callback();
}
```

Shortcut:
```javascript
function doSomeExpensiveOp(err, callback) {
  // Create and start the timer, and return the mutated callback
  callback = new RecordWork('do.some.expensive.op', callback).startWork();
  
  callback();
}
```