add a CocoaPods podspec file and push a version to pods repo

[mackuba]

Continuing the discussion from 72990b3:

I've looked into that and I think you should be able to just restore this deleted podspec, perhaps with just some minor updates, so I guess there's no point sending a PR with that.

Like I wrote there, pod specifications are now kept in the CocoaPods repo in a processed form (example: https://github.com/bahlo/SwiftGif/blob/master/SwiftGifOrigin.podspec -> https://github.com/CocoaPods/Specs/blob/master/Specs/SwiftGifOrigin/1.5.2/SwiftGifOrigin.podspec.json), and the process for releasing a new version is:

• tag a new release in your repo
• update the version number in the podspec file in your repo
• use the pod trunk push command to upload a new version of the spec to the cocoapods repo (after you've set it up once earlier)

So no need for manually preparing and sending a PR to the CocoaPods repo, you just run one command.

These guides should cover most of that:

• https://guides.cocoapods.org/making/making-a-cocoapod.html
• https://guides.cocoapods.org/making/getting-setup-with-trunk.html