Add date and date format validation rules

Adds `date` and `date_format:format` validation rules.  The date format string is tested against the string formatting options of http://php.net/manual/en/datetime.createfromformat.php

The following can be used as validation messages:
```php
	"date"            => "The :attribute is not a valid date.",
	"date_format"     => "The :attribute does not match the format :format.",
```

Tests also included.

I'm happy to update the docs if this is merged.