Add CocoaPods podspec.

This podspec should allow frankifying an app through CocoaPods, at least as far as I can tell. (I'm new to Frank.)

See also https://github.com/TestingWithFrank/Shelley/pull/18.

Unfortunately, there doesn't seem to be a clear separation between Mac-only and iOS-only files. For the future, it might make things easier to have separate subdirectories for these.

If accepted, I will also push this to the CocoaPods/Specs repo. But CocoaPods policy requires the versions to be tagged, so that would need to be done first.