Updated manifest and tab callbacks.

@alexnaspo

This should fix #13 and on my system resolves all errors in chrome.  It contains just some syntactical changes of `tabId` to `tab.id` and adds a call back function for the `var_dumplings.js` script.

```JavaScript
chrome.tabs.executeScript(tab.id, {
    file: "var_dumpling.js"
}, function(results){ console.log(results); });
```

Thanks for your review!