Update ClassMetadataInfo::wakeupReflection to handle nested embeddables.

Currently, it looks like nested embeddables are broken due to not handling this use case correctly in the ```ClassMetadataInfo::wakeupReflection``` method. This PR fixes this by reusing the ```ReflectionEmbeddedProperty``` and passing the already created sub-```ReflectionEmbeddedProperty``` on the ```ClassMetadataInfo``` of the subclass as ```childProperty``` instead of trying to create a plain ```ReflectionProperty``` with a multi-level property.

Note: passing the ```ClassMetadataFactory``` to ```wakeupReflection``` is probably a bit dirty, but this was the only solution I could so quickly think of. Note that the argument is optional to keep BC with other usages of ```wakeupReflection``` (it's public, so userland usages exist potentially). Of course, better suggestions are always welcome :-)

I'm not very familiar with Doctrine internals, so it could very well be I overlooked some part that might break with nested embeddables. Do you have any suggestions for areas should also be tested?