IntermediateSerializer Skeleton

This PR adds the initial skeleton of the various IntermediateSerializer classes as described from the XNA docs.  The implementation does not work yet.

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.