Simplify enum declarations

This pull request goes a bit further than @ohinckel's #178 and simplifies creating enums by providing the functionality (for the easiest case) in the base class:

Previous
--
```php
  uses('lang.Enum');

  class Weekday extends Enum {
    public static $MON, $TUE, $WED, $THU, $FRI, $SAT, $SUN;

    static function __static() {
      self::$MON= new self(1, 'MON');
      self::$TUE= new self(2, 'TUE');
      self::$WED= new self(3, 'WED');
      self::$THU= new self(4, 'THU');
      self::$FRI= new self(5, 'FRI');
      self::$SAT= new self(6, 'SAT');
      self::$SUN= new self(7, 'SUN');
    }

    public static function values() {
      return parent::membersOf(__CLASS__);
    }
  }
```

New
--
```php
  class Weekday extends \lang\Enum {
    public static $MON= 1, $TUE, $WED, $THU, $FRI, $SAT, $SUN;
  }
```

It also bumps the minimum requirement to PHP 5.3.0.