[cli] Added `Command`

Also added `MessageCommand`, but I’m not sure it will ever be needed. See it as a demonstration.
[cli] Fixed tests.
[cli/messages] Fixed path construction.
[cli] Improved help format and algorithm.
[messages.js] Excluded server-side messages from Ajax output
[languages] Added a way for getting all languages and their names
[Languages] Added tests
[Languages] Use Configuration::loadJson instead of local implementation