Check optional inputs only when they are defined

Hi,

Here is a pull request which adds the method `optional()`. It allow us to apply verification only if the element is not `undefined`.
This should closes #93 issue.

Example:
```javascript
req.checkBody('email').optional().isEmail();
//if there is no error, req.body.email is either undefined or a valid mail.
```

Tell me if you think I should change something.