Improve serialization for RProvider types

While this really seems to be an rdotnet issue, I thought it appropriate to create an issue here first.

When writing [this sample](http://mbrace.io/starterkit/HandsOnTutorial/examples/200-using-r-provider.html) for distributing RProvider code over mbrace, I noticed that many of the types exposed by the API happen to be non-serializable. I understand that most of these define wrapper objects around R values, so making them serializable should be a non-trivial yet workable task.