Add Podspec file to repository

This will allow people using GPUImage from Cocoapods (#749, #802, #1148, #1252, [etc.](https://github.com/BradLarson/GPUImage/search?q=cocoapods&type=Issues)) 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 => '...`.

All without requiring to set version tags (although they would still be very much appreciated!).