JSX Transformer / DisplayName Visitor: Multiple declarations with one `var` statement

The bug fixed by this commit prevented the correct parsing of `var` statements with multiple variables being declared. Instead of trying to parse a whole 'variable declarations' (a `var` statement with all its declarations), this visitor now only parses single 'variable declarators'.

An example (from the tutorial) that did not work before this patch:

```javascript
    var CommentBox = React.createClass({
      ...
    }),

    CommentList = React.createClass({
      ...
    });
```

What did  work is the following:

```javascript
    var CommentBox = React.createClass({
      ...
    });

    var CommentList = React.createClass({
      ...
    });
```

Of course, there are different coding styles and guidelines. Most developers probably don't like multiple variables being declared with one `var` statement, but nevertheless, it should be possible.