OpenPGP Provider API

I present the first version of my OpenPGP Provider API.
I would like to hear feedback and criticism on this pull request :)

Information:
- This API was discussed in recent posts on https://groups.google.com/d/forum/k-9-dev
- You can test this API by installing OpenPGP Keychain 2.1.1 from https://play.google.com/store/apps/details?id=org.sufficientlysecure.keychain (should be online in some hours)
- Support for APG is still working as before. It lives side by side with the new API
- OpenPGP apps can implement a service based on the definition and will automatically be shown in the preference list of available openpgp providers (no extra code per app!)
- Decryption/Verification is working automatically without buttons
- Different colors show different types of emails (decrypted-only=blue, signature-only verified=green, signature-and-decrypted verified=green, signature unknown=orange, error=red). This is similar to Enigmail

Todos:
- automatic sign and encryption is currently not working with the new provider