Add Cocoapods Podspec

[AndrewBarba]

I've already pushed the pod to Cocoapods under:

pod 'libCEd25519', '0.2.0'
pod 'Ed25519', '0.2.0'

No one should ever need to incluce libCEd25519 directly since Ed25519 depends on it and will be installed automatically.

If you merge this PR I'm happy to transfer the pods to your Cocoapods account. Otherwise if you release new versions I will just downmerge your changes and push from my fork

[vzsg]

Hi! Thank you for this.

I don't have a Cocoapods account AFAIK, nor am I currently planning to make one, so personally I'm fine with you managing it. If that helps, I can give you write access to this repo as well.

(I'll fix CircleCI later today, too.)

[AndrewBarba]

Honestly whatever is easier for you. If you update the version number in each .podspec when you add a new release to Github then I shouldn't need write access. I can just push to CP after the release is published. If its easier to give me write access thats totally fine too, I'll manage the podspec files for you