Logger: add #warning + doc.

* New method: `#warning`.
* Fix the tomdoc errors.
Setebos::Server: fix doc for `#remote`.
Setebos::Server: change `#send_files`.

* Do not log errors.
* Return an Array containing the files that can't be send.
* Doc.
Setebos::Run: new methods + send files.

* #validate_files_section: test correctness of the :files
section of the YAML config.
* #send_files: send files.
Use Setebos::Messages #1 in Setebos::Config.
Setebos::Server: remove `logger` require.
Setebos::VERSION: doc.
Use Setebos::Messages #2 in Setebos::Run.