#414 Refactor client and add documentation to the step interfaces

api/README.md

...tconnect/api/exceptions/client/AttachmentUploadError.java

...connect/api/exceptions/client/ClientNotAuthenticated.java

...onnect/api/exceptions/client/SubmissionCreationError.java

...fitconnect/api/exceptions/client/SubmissionSendError.java

...nect/api/exceptions/internal/AuthenticationException.java

...tconnect/api/exceptions/internal/DecryptionException.java

...tconnect/api/exceptions/internal/EncryptionException.java

...nect/api/exceptions/internal/InitializationException.java

...e/fitko/fitconnect/api/schema/FitConnectSchemaScopes.java

...rc/main/java/de/fitko/fitconnect/api/services/Sender.java

...ain/java/de/fitko/fitconnect/api/services/Subscriber.java

...a/de/fitko/fitconnect/api/services/auth/OAuthService.java

...e/fitko/fitconnect/api/services/crypto/CryptoService.java

...tko/fitconnect/api/services/metadata/MetadataService.java

client/src/main/java/de/fitko/fitconnect/TestRunner.java

client/src/main/java/de/fitko/fitconnect/client/Client.java

...rc/main/java/de/fitko/fitconnect/client/SenderClient.java

...ain/java/de/fitko/fitconnect/client/SubscriberClient.java

.../main/java/de/fitko/fitconnect/impl/SubmissionSender.java