Validator tag alias

We should add the ability to register a tag alias.
##### Suggestion

Make the Register method accept an `interface{}` and if it's a `string` then register it using an "AliasValidator".

```
validator.Register("port", "min(0),max(65535)")
validator.Register("ip4", "regexp(´\\b((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)(\\.|$)){4}\\b´)")
```
