Support bundled GPG and change API.

- Move openpgp encrypt/decrypt/sign/verify  API to inside OpenPGP class.
- Add encrypt/decrypt/sign/verify API to KeyManager.
- Add possibility of passing custom gpg binary to KeyManager and
  OpenPGPScheme.
- Remove "_asym" suffix from method names.
- Bump version to 0.2.1. New API is *not* backwards compatible.
Merge remote-tracking branch 'drebs/feature/3397-keymanager-should-support-bundled-gpg' into develop