Add OvershareKit podspec

I've started on an OvershareKit podspec as per #1.

This podspec lints cleanly with `pod spec lint` and I tried to address an unused variable warning. There's a few more steps I'd go through:

- [ ] Push a tag to OvershareKit, uncomment the `tag` line in the podspec, and remove the `commit` ref
- [ ] Add your email addresses to the podspec
- [ ] There are only stub subspecs for Pocket and ADN. I would create subfolders for each activity provider, which would allow you to include the `Pocket` and `ADN` files only with projects using those subspec(s), and might also help a bit with repo organization