Produce CSV string from Csv object and more

* Add a separate Csv.toCsvString to produce a CSV with all the contents of the Csv object (regardless of the length)
* Slightly change Csv.toString so it only shows "..." when there more lines that aren't being displayed
* Add constructor from List[List[String]]. Some alternatives:
  1. Do not use external library in the test, just pass
        List(
          List("ID", "Name,Value"), 
          List("A", "B,C"),
          List("D", "E,F"),
          List("G", "H,I")
        )
  2. Do not add this constructor at all, as it the same can be done with Csv(splitData.head, splitData.tail)
  3. Go all the way and remove any trace of CSV parsing from Planet7 (and rename Csv to SeparatedValues?)