Tagging Tensor with the Java serializable interface.

The FACTORIE Cubbie infrastructure directly stores Tensors as fields in the cubbie map, and unlike Cubbie, Tensor isn't Serializable. This makes it difficult to use FACTORIE with distributed computing platforms like Apache Spark, which require everything to be Serializable. Currently its possible to send most Cubbies over the wire (as they are extensions of Map), but it throws exceptions if they contain Tensors.

Tagging it with Serializable fixes this, and doesn't appear to have any side effects.