add podspec

Works in my project. 

You'll need to go through the CocoaPods guide on [setting up with Trunk](http://guides.cocoapods.org/making/getting-setup-with-trunk.html) to get it on to the main repo. Till then anyone can use `pod 'UIImageViewAligned', :git => 'https://github.com/orta/UIImageViewAligned.git'` to use my version.