Parse Github pull request labels from pull request payload.

[rtnpro]

As per https://developer.github.com/v3/pulls/#response-1, the pull request payload includes a field labels which contains a list of label map items.

However, it's not shown in the example: https://developer.github.com/v3/activity/events/types/#pullrequestevent for webhook events, although it references the pull_request object definition from https://developer.github.com/v3/pulls. I'd presume as a documentation bug on this Github help page.

This pull request adds support to parse Labels from a pull_request JSON object when unmarshalling pull_request event payload.

[rtnpro]

@joeybloggs ping! Could you review this change and let me know if it needs some more changes so that it can be merged?

[deankarn]

@rtnpro thanks for the ping, will merge asap