Add podspec

If you merge this Pull Request, I plan to submit a podspec to the Cocoapods Specs repo; however, the podspec won't pass `pod spec lint` until you do so (because LICENSE is missing).

Would you mind doing the following?
- [x] Choose a version (I chose 1.0.0 for you, but feel free to change)
- [ ] Tag the last commit in this Pull Request with the version
- [ ] Change the podspec to point to the tag instead of a commit