Update XML boolean format.

## XML Schema

http://www.w3.org/TR/xmlschema-2/#boolean
> An instance of a datatype that is defined as ·boolean· can have the following legal literals {true, false, 1, 0}.

## FuelPHP behaviour

```php
array('true' => true, 'false' => false)
```

### current format
```xml
<xml><true>1</true><false/></xml>
```
`false` is formatted to empty string.
It's PHP type casting works.
http://php.net/manual/en/language.types.string.php#language.types.string.casting

### new format
I add `bool_representation` option to change XML outputs.

If `bool_representation` is 
`true`:
```xml
<xml><true>true</true><false>false</false></xml>
```
`1`:
```xml
<xml><true>1</true><false>0</false></xml>
```
false value, such as `null`:
```xml
<xml><true>1</true><false/></xml>
```

Signed-off-by: Haruaki Otake <aaharu@hotmail.com>