Add a can_serialize_pb method to return TRUE if we will be able to
serialize this object as a protocol buffer.
Expose the new can_serialize_pb method.
Add a basic vignette showing how serialization of common datasets with
RProtoBufUtils compares to base R serialization.
Merge pull request #1 from murraystokely/master

Some docs and a can_serialize_pb method for RProtoBufUtils