Make JsonFactory Serializable

[cowtowncoder]

Usually we don't care whether non-persisted objects are Serializable. But in case of JsonFactory there are reasons to do this, for certain platforms. Specifically, on mobile platforms like Android, "freezing state" may require flushing out lots of things. And re-creating those things, conversely, may be more costly than would simple de-serialization be.

To see if this could work, it would be good to see how easy it'd be to make JsonFactory (which is the entry point to core streaming api) java.io.Serializable. And if that works, continue up the stack to databinding.

[cowtowncoder]

Implemented, added tests; works for stock standard JsonFactory with no ObjectCodec attached (that'll be second part). Needs to be done for ALL sub-classes however, to properly resolve.