Serializable should support `utf-8` encoding/decoding

[brimoor]

The eta.core.serial.Serializable class should support optional utf-8 encoding/decoding of strings. By default, I suppose this should be turned on.

Currently one has to write something like this:

output = etas.json_to_str(output).encode("utf-8")
with open(output_fn, "wt") as f:
    f.write(output)

[milosacimovic]

I second this