add cocoapods support

You should fork the [CocoaPods specs] (https://github.com/CocoaPods/Specs) GitHub repository and send a pull request.So this project can use cocoapods.

If only merge this pull request. you need to tell other user who want to use FESSolarCalculator  through CocoaPods by add 'pod 'FESSolarCalculator', :git=> 'https://github.com/danimal/FESSolarCalculator.git' to the READMME.md.