Add Webhook and functions: clearHistory, getMessage

[ged-nn]

Add RocketChatWebHook.php

    This need for work as Outgoing WebHook.
        * __construct - get post data as array $postData from json and $text as array from text message
        * function sendmessage() - send message to current channel

README.md

    + example usage WebHook

RocketChatChannel.php

    + clearHistory() - Clean history of the channel.

RocketChatClient.php

    + getMessage() - Get message by msgId. Need access for user to room or chat.