Add comment support

This commit adds the ability to read and generate comments on the zip file and/or on each zip entry, see #134.
The entry comment is set on `ZipObject#comment` and the zip file main comment is set on `JSZip#comment`. For example :

```js
var zip = new JSZip(data);
zip.comment // contains the comment of the loaded zip
zip.generate({type:"blob", comment: "other comment"}); // generate a zip with a new comment

zip.file("Hello.txt").comment // contains the comment of the loaded entry, if any
zip.file("World.txt", "content", {comment : "comment here"}); // create a new file with a comment
```

I also had issues with saucelabs tests and I resolved them with an upgrade of the grunt plugin (I also upgraded other dev dependencies).