Validation of nested arrays, closes #8

This pull request extends Valitron to allow validation of multidimensional arrays, as well as validating all members of an array by passing an asterisk. I've also added an "array" rule that validates that a field is an array.

Examples:

```php
$v->rule('array', 'examples');
$v->rule('array', 'examples.*');
$v->rule('boolean', 'examples.*.enabled');
```