GitHubPayloadAnalyzerConfiguration::getDefaultGroup()

Summary:
The responsibility to determine the default group were located
inside GitHubPayloadAnalyzer.

As, this is a pure configuration setting, the responsibility is
moved to GitHubPayloadAnalyzerConfiguration.

A constructor is added, to specify project, so we can do two things:

• in this change, determine a fallback default group when omitted from the configuration
• in a next change, give it the responsibility to load JSON config

Test Plan: phpunit

Reviewers: dereckson

Subscribers: alken-orin

Differential Revision: http://devcentral.nasqueron.org/D280