Newrelic custom measurements utility

Our use case:

```php
$measurements = CustomMeasurements::create("LinkSuggest");
$measurements->measureTime( "getLinkSuggest", function() use($measurements) {
	/// do some stuff

	$results1 = $measurements->measureTime( "getLinkSuggest-sql1", function() { 
		$results = doSomeQuery();
		return $results;
	});

	$results2 = $measurements->measureTime( "getLinkSuggest-sql2", function() {
		$results = doSomeOtherQuery();
		return $results;
	});
	/// do some more stuff with results (or something)
});
```

@Szumo @federico-lox @wladekb