Adds podspec

Also updates README instructions for installation. 
Haha, I was hoping you could tell me! :laughing: 

Honestly, I've always been murky on the subject. From what I hear, Swift libraries can target iOS 7.0+ and OS X 10.9+, so I guess we should go with those?

But I'm pretty sure our CI only tests the latest, i.e.: 8.0 and 10.10--should we be concerned? I don't want to promise support for targets that aren't tested.

We could just play it safe with 8.0 and 10.10, and update the podspec/CI if someone requests support for older targets.