Introduce podspec and CocoaPods instructions

I've only added @gfontenot as an author as I couldn't find @tonyd256's full name or email.

One of the authors should `pod trunk push Argo.podspec` for the instructions in the README to work. Or the instructions should read:

```ruby
pod 'Argo', :git => 'https://github.com/thoughtbot/Argo'
```
Other authors:

`Tony DiPasquale => tony@thoughtbot.com`
`thoughtbot` (no email)

I don't remember if it's possible to mix and match authors with emails and authors without emails. If not, it's totally cool to omit Tony and my emails from the author list.