Add comments to IncomingDataTransferCommand

The IncomingDataTransferCommand class was missing some JavaDoc comments.
This commit adds these comments.
Add toString() to IncomingDataTransferCommand

The IncomingDataTransferCommand class didn't contain a specific
toString() implementation. This commit adds this implementation.
Add DataTransferMessage to event

Refactored the IncomingDataTransferReceivedEvent to use the new
DataTransferMessage type instead of multiple simple fields.
Remove file appender from test logging

The Logback configuration for test runs contained a file appender which
caused log files to be written to the project's directories. This commit
removes the file appender which causes the log files to no longer
appear when building Motown.
Use default DataTransferMessage in unit test

The IncomingDataTransferCommandTest didn't use the test utilities
default DataTransferMessage. This commit changes that and simplifies the
tests by using the default test data.
Refactor RequestDataTransferCommand

Refactored the RequestDataTransferMessage to use the new
RequestDataTransferCommand. Some additional changes include:

- Renamed the unit test to match the subject's name.
- Changed the JSON API to match the new core API.