Refactor GitHubPayloadAnalyzer into payloads individual rules

Summary:
Instead of trying to parse every payload in GitHubPayloadAnalyzer,
we split the processing into several rules class, one class per type.
They are located in app/Analyzers/GitHub/Events.

So, if GitHub introduces a new event type, the only thing to do is
to create a new class for this event, provide sensible return values
for getDescription and getLink. The class will then be autoloaded.

Test Plan:
phpunit

Code coverage at 100% for the new classes and the
modified parts of GitHubPayloadAnalyzer.

Reviewers: dereckson

Subscribers: alken-orin

Differential Revision: http://devcentral.nasqueron.org/D274