Adding PodSpec for CocoaPods repository

Hi, 

Not sure if you know, but there's currently a podspec for UIAlertView-Blocks on CocoaPods (http://cocoapods.org/?q=name%3Auialertview), though I've noticed it hasn't been updated in over a year.  

I've just used the existing podspec on the CocoaPods site, but updated it to the latest commit in this repo, and sent them a pull request to include it (https://github.com/CocoaPods/Specs/pull/3640).

You don't really need to check this file in, though it would be handy to have it around so the next person who updates the podspec can just re-use this file.

Thanks!