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 ;-)