Add new GitHub parser called LabelUpdateEventParser.

[BURG3R5]

This issue depends upon the issues #7 and #8 and thus can be only attempted after those issues are closed.

In this issue, you'll add a new GitHub parser class called LabelUpdateEventParser in the file bot/github/github_parsers.py. The data sent to us by GitHub for this event can be found here.

Step 1

Create LabelUpdateEventParser class.

For this step, you'll do the following:

1. Create a class called LabelUpdateEventParser that inherits from EventParser and implements the static methods verify_payload and cast_payload_to_event just like the other parsers in the file.
2. In the verify_payload method of this class, the condition should just check that event_type == "label". Nothing else is required.
3. In the cast_payload_to_event method, event_type, repo and user should be assigned properly according to the other parsers. Then, label should be assigned to a Label object with the name and action being set to json["label"]["name"] and json["action"], respectively.

Step 2

Update list in parse method

For this step, all you need to do is go to the list event_parsers inside the parse method inside the GitHubPayloadParser class, and then add LabelUpdateEventParser at the end of the list.

This issue is a part of a bigger issue (#6).
Leave a comment if you have any doubts.