[NEW] Query the GPG-Agent for a passhrase from within a sandbox.

- Added isPassphraseForKeyInGPGAgentCache:(NSString *)key reply:(void (^)(BOOL))reply
  to the XPC service.
- Moved necessary methods from GPGController into GPGTaskHelper,
  otherwise it would be necessary, to build all sources of Libmacgpg
  into the XPC service.
Removed some debug logging.
Link Libmacgpg.framework into ~/Library/Framework for development.
Merge branch 'dev' of https://github.com/GPGTools/Libmacgpg into dev