Added error handeling in stat methods

Right now there's no way of catch errors in the stat methods. With this commit we will be able to use the traditional way of callbacks:

```js
racoon.mostLiked(function(results){
  /*logic here*/
})
``` 

Or use a function of arity 2 to handle the errors:
```js
racoon.mostLiked(function(err, results){
  if(err) /*something*/;
  /*logic here*/
})
```
This is a step in the issue #10