Add sanitize attribute to clean object

Cleans object, removing non-matching properties.

See bellow:

```
// Schema
{
   str: { type: 'string' },
   sanitize: true
}
````

```
// Object
{
   str: 'hi',
   extra: 'unimportant data, discard this',
   extra1: 'discard this'
}
```

```
// Result object (after validate):
{
   str: 'hi'
}
```

```
// Getting result object
newObject = schema.validate(Object);

// or
schema.validate(Object, function(err, obj) {
   newObject = obj;
});
```