Embeddables in metadata builder

Embeddables set the `$classMetadata->isEmbeddedClass = true` and sets `$classMetadata->isMappedSuperclass = false`, imitating the `switch` in xml / yml drivers.

Embeddeds have 2 methods, as other types have: `addEmbedded` is a one-off adder with fluent behavior, and `createEmbedded` returns an instance of `EmbeddedBuilder`. The class is very thin right now, but it's a good way to lay ground for improvements on embedded creation in the future.

This one is probably more useful than the last one, @Ocramius ;-)
XmlMappingDriverTest.php

```
public function testEmbeddableMapping()
    {
        $class = $this->createClassMetadata('Doctrine\Tests\Models\ValueObjects\Name');

        $this->assertEquals(true, $class->isEmbeddedClass);
    }

    public function testEmbeddedMapping()
    {
        $class = $this->createClassMetadata('Doctrine\Tests\Models\ValueObjects\Person');

        $this->assertEquals(
            array(
                'name' => array(
                    'class' => 'Doctrine\Tests\Models\ValueObjects\Name',
                    'columnPrefix' => 'nm_',
                    'declaredField' => null,
                    'originalField' => null,
                )
            ),
            $class->embeddedClasses
        );
    }
```