feat(screenshots): add screenshot feature

Add screenshot feature based on [html2canvas](https://github.com/niklasvh/html2canvas). As library stated supported browsers are (needed to be tested):

* Firefox 3.5+
* Google Chrome
* Opera 12+
* IE9+
* Safari 6+ 

To test if it's working uncomment **src/app/services/services.js:45**:
```
document.body.appendChild(screenshotImage);
```

TODO test and check error scenario (no image generted).