save methods

Adding three methods to the API:

* saveTable()
* saveJSON()
* saveStrings()

As part of this process, I also worked on the PrintWriter class, which formats output, the writeFile() method, which prompts download, and some more helper methods like escape(). I hope these'll come in handy down the line !
Sounds good. Should the p5.PrintWriter be publicly accessible (and documented for the API?) or would it be better to make it private, like p5._PrintWriter ?