Add screenshot to the Podspec

[fabiopelosin]

Hi, it would be great if you could add a screenshot to the podspec and push the update to the specs repo. The screenshot is currently displayed on http://feeds.cocoapods.org and in future it will displayed on the search of http://cocoapods.org.

To learn more about the screenshot attribute, you can check the documentation here: http://guides.cocoapods.org/syntax/podspec.html#screenshots