Qr Code generation

Hi, it's been a while

This PR provide Qr Codes generation inside pdfMake. a new node type `qr` is provided

```javascript
 // pass text as argument
 { qr: 'Can you see me'},
   
// options
//- version: an integer in [1,40]. when omitted (or -1) the smallest possible version is chosen.
// - mode: one of 'numeric', 'alphanumeric', 'octet'. when omitted the smallest possible mode is chosen.
// - eccLevel: one of 'L', 'M', 'Q', 'H'. defaults to 'L'.
// - mask: an integer in [0,7]. when omitted (or -1) the best mask is chosen.
// -fit : resize to fit the provided size
{ qr: 'http://pdfmake.org/.', fit: 120, errLevel: 'Q', alignment: 'right' },
```

work was bases upon a tweaked version of the [neocotic.com/qr.js](http://neocotic.com/qr.js/). But instead of taking the long way of generating an image and embed it in you docDef; this PR uses the generated frame Data and convert it to vector data usable by pdfMake