Add Podspec file

This will allow people using GPUImage from Cocoapods to:
* Force the latest version of the library at any time with `pod 'GPUImage', :head`.
* Specify a given commit and/or fork to use with `pod 'GPUImage', :git => 'https://github.com/...', :commit => '...`.
Merge pull request #1270 from rivera-ernesto/add_podspec

Add Podspec file to repository