Export ValidationException

I've changed ValidatoinException [to "inherite" from Error](http://stackoverflow.com/questions/8802845/inheriting-from-the-error-object-where-is-the-message-property)
And exported it out

Now it can be possible to handle error like:
```javascript
try {
  is(123).string().throwError()
} catch (err) {
  if (err instanceof is.ValidationException) { ... }
  else { ... }
}
```