Refactor GitHubPayloadAnalyzer into payloads individual rules
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.
Code coverage at 100% for the new classes and the
modified parts of GitHubPayloadAnalyzer.
Differential Revision: http://devcentral.nasqueron.org/D274