(Very) Basic LINQ support against BsonDocumentBackedClass

This is a very simple change that I made as part of our current project to allow us to do basic linq queries against weakly typed documents. The limitation is that it does not support nested documents so you can only write queries against bson elements in the root.

I don't know if this is something that you would consider including as it may have implications for other functionality that I'm not aware of, but as it was such a small change I thought I'd send a PR anyway!

Also apologies for the line endings but I did not notice until I pushed.

Predicate Translator\BsonInfoFinder will now attempt to look for a BsonDocumentSerializer in VisitGetItem.