In a paragraph with CRLF line endings, Livy interpreter causes HTTP 500 status

This issue is particularily noticeable when a Zeppelin user runs their browser on a Windows system, where copy-pasting code (e.g. from a Jupyter notebook) may result in the situation where multi-line paragraphs end up containing CRLF characters. Executing the paragraph fails with only a cryptic "500 Internal Server Error" message as the only clue in the notebook.

As it turns out, these HTTP 500 errors were generated by Livy. The logs of the Livy server reveal that "CTRL-CHAR 13" is being rejected by the JSON parser is the cause for these errors. When investigating the cause for this, `org.apache.zeppelin.livy.LivyHelper` turns out to be the culprit. Among the many other problems this class has, it does not attempt to escape the required characters in the interpreter input when generating the JSON payload for the POST request it sends to Livy.

Incidentally, a quick and dirty patch seems to be incorporated in the patch for ZEPPELIN-1430. However, that patch targets version 0.7.0, while I expect that this issue affects all of 0.6.