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 callback 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!

-EDIT

This also includes changes to `permissions` in `manifest.json` using `activeTab` instead of `tabs`.  Both will work, but I felt `activeTab` was sufficient with `*://*/*` for backwards compatibility.