Fix stdout / stderr so they come back correctly as side effects and not as data values

[apatrida]

Currently after PR #5 the stdout comes back as a string data value, should be changed to be just side effect STDOUT output. It works in console but not in notebooks, so obviously isn't correct yet.

Get errors and stdout to work the same as Python or Scala does within the notebooks.

[apatrida]

The issue might be that these need to be in reply to the execute request and not just sent at random times for them to appear.