Fixed PodSpec

Hi Francesco,

First off, thanks for the great work on FDStatusBarNotifierView! Thanks also for including a podspec in your repo. Unfortunately, your podspec is not currently a part of the [official Specs repository](https://github.com/CocoaPods/Specs), and when I tried to add it, there were some validation issues with the podspec itself.

I've fixed these issues, and have added a tag to the repo as required by CocoaPods. Please merge this pull request; once you do, I'll be able to publish your podspec to the [official Specs repository](https://github.com/CocoaPods/Specs), and then anyone using CocoaPods can easily find and use your code.

Thanks again for your work on FDStatusBarNotifierView; let me know if you have any questions.

Thanks!
Matthew