[Feat] Bot should send commands via private message

[JorgeMiguelGomes]

In order to avoid cluttering channels with information, when a user sends the "?commands" command, the bot should:

• reply to the user that the command's list was sent via private message
• send commands' list via private message

[miguelcarreiro]

Issue fixed in commit https://github.com/vostpt/bot/commit/57760969c5059aca56d79b67756258d860652a46, and PR https://github.com/vostpt/bot/pull/75