Saddle data types should be serializable 

In order to use Saddle data types in a context where java serialisation is necessary (e.g. when a data processing framework is "checkpointing" in-progress results during a computation made of many steps), I updated the Vec, Index, Series, Mat and Frame classes/traits so they are now serialisable. Class members have been marked as "transient lazy" whenever re-creating them is possible based on other piece of class state. 

I also added unit tests that make sure no data is lost after a serialisation/deserialisation round trip. 

Build number has been increased to 1.3.4-SNAPSHOT.