Exclude functions from cloned meta objects

When we add some custom function to Object.prototype, winston logs that code as well since clone function copies everything from meta object. E.g. cloned {} meta will have Object.prototype functions which is quite useless in meta I believe.

Resolves #420 

Issue:

Defining new Object function:
```javascript
Object.prototype.hasOwnValue = function(val) {
	for(var prop in this)
	{
		if(this.hasOwnProperty(prop) && this[prop] === val)
		{
			return true;   
		}
	}

    return false;
};
```

Writing some text to log without meta:
```javascript
logger.log("warn", "Logger initialized");`
```

And the result:
![image](https://cloud.githubusercontent.com/assets/3230176/4178043/17d39bf8-3673-11e4-9850-e298957264b5.png)