BuildableFromConfigInterface

Summary:
makeFromConfig static method is now defined in an interface, called
BuildableFromConfigInterface.

This solves the followig issue introduced at rKBROKER0bce735feb1b:

Static function Keruald\Broker\Broker::makeFromConfig()
should not be abstract

Test Plan: Run tests here and from Notifications center

Reviewers: dereckson

Differential Revision: http://devcentral.nasqueron.org/D227