Add podspec for CocoaPods

It would be nice to have tags on the repo in order to identity future releases. By default, CocoaPods uses `0.0.1`, but that’s really not ideal.

---

[Semantic version](http://semver.org) tags (instead of plain commit hashes/revisions) allow for [resolution of cross-dependencies](https://github.com/CocoaPods/Specs/wiki/Cross-dependencies-resolution-example).

In case you didn’t know this yet; you can tag the current HEAD as, for instance, version 1.0.0, like so:

```
$ git tag -a 1.0.0 -m "Tag release 1.0.0"
$ git push --tags
```