IntermediateSerializer Skeleton

This PR adds the initial skeleton of the various IntermediateSerializer classes as described from the XNA docs.

I'm also starting to port over the work @JamesLupiani  did in the XmlImporter (see https://github.com/mono/MonoGame/pull/2380) which basically implements what the IntermediateSerializer classes need to do.

Eventually we should be able to replace the current `XmlImporter` implementation with a single call to `IntermediateSerializer.Deserialize()`.  This is in fact how i'm testing that this works.

My next step is to implement the overloaded `ContentTypeSerializer` which is what seems to drive the entire XML serialization process.  This looks like a very similar design to how the `ContentTypeWriter`/`ContentTypeReader` system works.