Create RuleName rule

Use validators by using its name:

```php
v::ruleName('int')->validate(42); //true
```

The best think on this rule if that you also can append or prepend custom prefixes to use your own rules:
```php
Respect\Validation\Rules\RuleName::prependPrefix('My\\Priority\\Rules\\');
Respect\Validation\Rules\RuleName::appendPrefix('My\\Other\\Rules\\');
```

Related to #176